Correct option is D
Given: 31, 29, 24, 22, 17, (...)
31 to 29: Subtract 2
31−2=29
29 to 24: Subtract 5
29−5=24
24 to 22: Subtract 2
24−2=22
22 to 17: Subtract 5
22−5=17
It alternates between subtracting 2 and subtracting 5.
Continue the pattern:
From 17, subtract 2 (next in the pattern):
17−2=15
So, the missing number in the series is 15.
The correct answer is (D) 15.