(a) 96.
The thousands’ place cannot be occupied by 0, as the number will then become three digit, so there are 4 choices for thousands’ place. As repetition is not allowed, so leaving the digit occupying thousands’ place, there are again 4 choices for hundreds’ place (including 0)
Similarly for Tens’ place there 3 choices, for units’ place 2 choices.
∴ Total number of 4-digit numbers without repetition of digits = 4 × 4 × 3 × 2 = 96.