A number can be expressed as a sum of consecutive natural numbers if and only if it is not a power of 2. Powers of 2 (2, 4, 8, 16, 32, …) cannot be written in this form.
- 21 = 10 + 11
- 29 = 14 + 15
- 45 = 22 + 23
- 32 = 25 (a power of 2), so it cannot be expressed as a sum of consecutive natural numbers.
Correct answer: C) 32