For a mod N counter the number of flip flops required is less than or equal to 2 raised to power n where n is a positive integer.
N<= 2^n
Hence,
For a mod 10 counter, 10< 2^4. So 4 flip flops are required.
For a mod 16 counter, 16=2^4. So again 4 flip flops are required.
For a mod 19 counter, 19<2^5. So again 5 flip flops are required.
For a mod 32 counter, 32=2^5. So 5 flip flops are required.
Thus 5 flipflops are required.
Option B. 5
