Correct option is C
Given:
- Initially, A has 2 litres of water; B is empty.
- Every hour: 1 litre is transferred from A to B.
- After half an hour: 0.5 litre is returned from B to A.
Step-by-Step Process:

Final Answer:
(C) 3 h
A becomes completely empty at the end of the 3rd hour.