Which of the following is a prime number?

Given data

• Options: 31, 51, 91, 121.

Concept / Approach

• Prime test up to √n.

Checks

31: √31 ≈ 5.57; not divisible by 2, 3, 5 ⇒ prime51 = 3 × 17 ⇒ composite91 = 7 × 13 ⇒ composite121 = 11 × 11 ⇒ composite

Common pitfalls

• Forgetting 91 = 7 × 13 (often mistaken for prime).

Final Answer

31