system
Given the following post, respond as if you're commenting on Reddit:
human
[Mass Effect] In 2013, humanity recieves a chilling premonition...(scenario inside)

The date's today, September 19th, 2013. Everyone is going about their daily business. In a dig site in Egypt, some archaeologists find a mysterious capsule. There is a small button on the side that they press. All of a sudden, everyone on Earth is contacted via some sort of telepathy. Everyone hears it, no matter who they are.


The speaker reveals themselves, to the shock of the world, as Protheans from Mass Effect. The telepathic message has everyone see that it is indeed a trio of Protheans. They display proof in the form of memories from the empire. For the rest of the message, the Protheans leave an omninous warning; the Reapers are real and they are coming. We have a set amount of time until they arrive from Dark Space. We need to prepare. Upon the completion of their harvest, the will retreat back into dark space and return in 46,736 years. (Assume that the Vanguard does not react to technological progress and prematurely summons them)


What could we do to survive?


Scenario 1: They will arrive in 20 years

Scenario 2: 50 years

Scenario 3: 100 years

Scenario 4: 500 years

Scenario 5: 1000 years

Scenario 6: 5000 years


Bonus Scenario: The Prothean Archives are exactly where they were under Promethei Planum, just like in canon. Additionally spread throughout the galaxy are anywhere from 1-2 million arguable Prothean ruins that can range from unfinished outposts to shipyards (all heavily damaged of course). Locations to a good amount could be accessed from the archives, but will require immensely powerful computers to crack. (perhaps a few decades away).


Rules:
1: No other Mass Effect species, just Protheans and Reapers.

2: It's unknown whether there are space faring civilizations, but if the Reapers are coming, than there could be.

3: All feats of the Reapers in Mass Effect are their real life capabilities.

4: For this scenario, the Reapers consider any species that has entered space as "space faring". So it won't matter whether the race has a multi-system empire or are launching communication satellites, the Reapers will go after them regardless. Meaning even modern Earth would be under threat.

EDIT: For the bonus scenario, the relays are still around, with the Sol relay still in Charon. Eezo is limited, but exists.
gpt
For scenario's 1 and 2 our best hope would be to pull a Raloi and destroy all our satellites and any evidence we are a space faring race. But after 50 years, that will no longer really be an option.

For Scenario 3 we still are not capable of taking on The Reapers, but at this point we have no choice but to be Space Faring.  For this scenario, we would probably send out hundreds of colony ships. However, their destination is determined once they have left Earth, all records of their departure are destroyed, and the colony can never make contact with Earth until ten years after the last sighting of a Reaper. Each ship would have the equipment to build a massive underground bunkers that would be equipped with Cryo Pods and Cloning Tech. When Reapers are first seen all colonists go into Cryo Sleep until it is deemed safe enough to come out by the AI overseeing the colony. The Cloning Tech is there in case of some catastrophic failure and all the colonists die in Cyro. Once this happens, the AI would begin pumping out clones and then raise them and teach them. This will ensure that it will be almost impossible for The Reapers to find every single colony, and there are contingencies in place to maximize our survival.

For Scenario's 4,5, and 6 we are now able to take The Reapers on in a head-on war. Since Humanity gained a lot of knowledge on The Reapers from the Protheans, they should know that the Citadel acts as a massive Mass Effect Relay for the Reapers. Since all the Reapers will warp their when they first arrive, Humanity can gain a huge edge if they set some basic traps like minefields, an ambush with thousands of ships, throwing some asteroids at them, or even rigging the Citadel to explode, taking a good chunk of The Reapers out with it. Lots of options.