How many 4-digit personal identification numbers are possible if the number cannot contain a zero?

Each of the four positions can contain any of the 9 nonzero digits (1 through 9). Since the question does not forbid repeating digits, there are 9 × 9 × 9 × 9 = 6,561 possible PINs. The correct choice is 6,561.

AI-written and checked against the verified answer.