Fill the 6-liter container first,  and then pour the water from the 6-liter container into the 5-liter container.  After the 5-liter container is filled, there is 1 liter of water left in the 6-liter container.  It takes two operations.  So the answer is $$A$$ 
There are two containers, one with a capacity of 6 liters and the other with a capacity of 5 liters, and using them to get 1 liter of water from a bucket requires at leasttimes of operations. 
  * $$2$$ 
  * $$3$$ 
  * $$4$$ 
  * $$5$$ 