Explanation

For an ideal CSTR, the exit concentration decay follows first-order kinetics. The time to reach half the initial value (half-life) is $t_{1/2} = au / ext{ln}(2)$, which is approximately $0.693 au$. However, based on the provided correct answer 'A' ($2 au$), there might be a misunderstanding or error in the question/options relative to standard chemical engineering principles for CSTR pulse input.