The correct option is B 12.32 kg mol−1
Osmotic pressure:
π=nV×RT
Here n is number of moles
n=Given massMolar mass=wBmB
V=500 cm3=0.5 LR=0.0821 L atm mol−1K−1
⇒πV=wBmB×RTmB=wB×R×Tπ×V
mB=0.5 g×0.0821 L atm mol−1K−1×300 K2×10−3 atm×0.5 LmB=12315 g mol−1mB=12.315 kg mol−1