Right answer is (b) t+\(\frac{\Delta t}{2}\)
The explanation is: The first step of the ADI scheme is from t to t+\(\frac{\Delta t}{2}\). The second step of the ADI scheme is from t+\(\frac{\Delta t}{2}\) to t+Δt. Here, the intermediate step t+\(\frac{\Delta t}{2}\) is extra for the process and the results here are not actually needed.