Choose the number which is different from the others in the group.

161

The correct answer is Option (2) → 161

Given numbers: 131, 151, 161, 171

Examine the middle digit of each number:

• 131 → middle digit: 3 (odd)
• 151 → middle digit: 5 (odd)
• 161 → middle digit: 6 (even)
• 171 → middle digit: 7 (odd)

Only 161 has an even middle digit; all others have odd middle digits.