ANS:D - 4

Decade counter counts from 0-9 i.e 0000 to 1001.

How many FF required means 2^n FF.
Suppose if we use 3 FF i.e 2^3=8 states (000 to 111).
Decade counter means total of 10 (0 to 9).
Therefore we require 4FF (2^4=16 states).