Q.Pressure cooker reduces cooking time for food because
a. boiling point of water involved in cooking is increased
b. heat is more evenly distributed in the cooking space
c. the higher pressure inside the cooker crushes the food material
d. cooking involves chemical changes helped by a rise in temperature.

Boiling point elevation is the rise in a solution's boiling point above the pure solvent's boiling point, caused by dissolving a nonvolatile solute: $\Delta T_b=T_b-T_b^0$. It follows directly from vapour pressure lowering -- since the solution's vapour pressure is lower than the pure solvent's at every temperature, the solution's vapour-pressure-vs-temperature curve sits below the solvent's curve, so a HIGHER temperature is needed for the solution's curve to reach the 760 mm (1 atm) line where boiling occurs. Experimentally, $\Delta T_b$ is directly proportional to molality: $\Delta T_b=K_bm$, where $K_b$ (the ebullioscopic or molal elevation constant) is a property of the solvent alone, numerically equal to the elevation a 1-molal solution would produce. This becomes a molar-mass-determination formula: substituting molality's definition ($m=1000W_2/(M_2W_1)$) gives $M_2=\dfrac{1000K_bW_2}{\Delta T_bW_1}$ …