Final answer:
The number with exactly two factors is 7, which is a prime number. The factors of 7 are 1 and itself, making it the only number in the list that meets the criteria of having exactly two factors.
Step-by-step explanation:
A number that has exactly two factors, one and itself, is a prime number. 7 is a prime number because it can only be divided evenly by 1 and 7. 8 has factors 1, 2, 4, and 8. Therefore, it has more than two factors. 9 has factors 1, 3, and 9. Therefore, it has more than two factors.10 has factors 1, 2, 5, and 10. Therefore, it has more than two factors. Therefore, the correct answer is a) 7, which has exactly two factors: 1 and itself.