The correct option is A 73.9 kg mol−1
Osmotic pressure:
π=nV×RT
Here n is number of moles
n=Given massMolar mass=wBmB
V=200 cm3=0.2 LR=0.0821 L atm mol−1K−1
⇒πV=wBmB×RTmB=wB×R×Tπ×V
mB=1.2 g×0.0821 L atm mol−1K−1×300 K2×10−3 atm×0.2 LmB=73890 g mol−1mB=73.89 kg mol−1