The condition under which a microwave oven heats up a food item containing water molecules most efficiently is

the frequency of the microwaves has no relation with natural frequency of water molecules. microwaves are heat waves, so always produce heating. infra-red waves produce heating in a microwave oven. the frequency of the microwaves must match the resonant frequency of the water molecules.

the frequency of the microwaves must match the resonant frequency of the water molecules.

Required condition : Frequency of microwaves = Resonant frequency of water molecules.