The correct option is B 5.77 K kg mol−1
Since,
Freezing point depression constant,
Kf=RT2f1000×Lfwhere,R=8.314 J K−1mol−1Freezing point of pure solvent, Tf=−23∘C=273−23=250K,Latent heat of fusion, Lf=90 J g−1
Substituting the values in the above equation,
Kf=8.314×(250)21000×90=5.77 K g mol−1