Find out the odd word/letters/number/number pair from the given alternatives.

Concept: A number is divisible by 9 if and only if the sum of its digits is a multiple of 9 — this is the standard divisibility rule for 9.

Application: Applying this rule to each alternative:

1. 72 → 7 + 2 = 9, a multiple of 9, so 72 is divisible by 9.

2. 45 → 4 + 5 = 9, a multiple of 9, so 45 is divisible by 9.

3. 81 → 8 + 1 = 9, a multiple of 9, so 81 is divisible by 9.

4. 28 → 2 + 8 = 10, not a multiple of 9, so 28 is not divisible by 9.

Cross-check: Direct division confirms this: 72 ÷ 9 = 8, 45 ÷ 9 = 5, and 81 ÷ 9 = 9, all exact, while 28 ÷ 9 leaves a remainder.

Since three of the four numbers are divisible by 9 and one is not, the number that does not fit the pattern — 28 — is the odd one out.