The correct option is C 342.4 g/mol
Freezing point depression of a solution is given by,
△Tf=Kf×m
where,
△Tf is the depression in freezing point.
Kf is molal depression constant.
m is molality of the solution.
Molality=△TfKf
Molality (m)=wsoluteM×1000Wsolvent
∴
△TfKf=wsoluteM×1000Wsolvent
where,
w is weight of solute
W is weight of solvent
M is molar mass of the solute
M=1000Kf×wW×△TfGiven, Kf=1.863 K kg mol−1w=13.6 g,W=20 g,△Tf=3.7∘CM=1000×1.863×13.620×3.7⇒342.39
Molar mass of the substance is 342.4 g/mol