The effective length of prismatic steel member of length L in compression having both its ends restrained is:

Correct option is D

For a prismatic steel compression member with both ends restrained (fixed–fixed condition):
End restraints significantly reduce buckling tendency.
As per IS 800 (Steel Structures), the effective length factor (K) for a fixed–fixed column is 0.65.
Effective length = K×L = 0.65L