The strong acid would completely dissociate, at least at first. This would lead to a pH of 2 in the case of the liter of pure water, while the pH would drop, but not nearly as much, for the liter of acetic acid. This is because acetic acid is a weak acid which acts as a buffer, taking up protons from solution. The dissociation of acetic acid is pushed back towards the free acid away from the anionic form of the acid which has lost a proton.
Work Step by Step
This question is asking you to see the difference between water and a solution of a buffer (acetic acid) in water, in terms of how much pH changes when acid is added.