We need to make units same first. $5$ minutes equal to $300$ seconds. Now we could remove the same unit, second. We get $300:30$ and simplify it to $10:1$. 
What is the simplest form of $5$ minutes$: 30 $ seconds? 
(A) $5:30$ 
(B) $1:6$ 
(C) $6:1$ 
(D) $10:1$ 