Is the number z even? a) z is divisible by 4 b) z is divisible by 3

Concept: In a data-sufficiency question, a statement is sufficient only if it forces exactly one definite answer to the question asked. For parity (odd/even), recall that since 4 = 2 x 2, every multiple of 4 is automatically a multiple of 2 -- so divisibility by 4 guarantees an even number. Divisibility by 3 carries no such guarantee, because multiples of 3 include both even values (6, 12, 18) and odd values (3, 9, 15).

1. Statement a) says z is divisible by 4. Every multiple of 4 (4, 8, 12, 16, ...) is even, so this statement fixes z as even on its own -- Statement a) alone is sufficient.

2. Statement b) says z is divisible by 3. Multiples of 3 include even numbers (6, 12, 24) and odd numbers (3, 9, 21), so knowing only that z is divisible by 3 does not fix its parity -- Statement b) alone is not sufficient.

3. Since one of the two statements already answers the question by itself, there is no need to test whether combining both statements is required, and there is no need for both statements to work individually for an 'either' verdict to hold.

Cross-check: pick a value consistent with Statement a) only, such as z = 8 -- divisible by 4, and indeed even, confirming sufficiency. For Statement b) alone, z = 3 (odd) and z = 6 (even) both satisfy 'divisible by 3', so this statement cannot fix parity by itself, confirming insufficiency.

Result: Statement a) alone answers the question, while Statement b) alone does not -- so 'Statement A alone is sufficient' is the correct choice.