Identify the relevant interaction

A microwave oven uses electromagnetic waves of frequency about $\nu \approx 2.45\,\text{GHz}.$

This lies in the microwave region of the electromagnetic spectrum.

How microwaves heat food

Water molecules are polar molecules. The alternating electric field of the microwave causes these dipoles to try to align repeatedly with the field.

This produces mainly rotational motion of water molecules, and due to collisions, this rotational energy gets converted into thermal energy.

Check each option

• A: transferring electrons from lower to higher energy levels in water molecule
  This is an electronic transition, usually caused by much higher-energy radiation like UV/visible, not microwaves. So this is incorrect.

• B: giving rotational energy to water molecules
  Microwaves primarily interact with polar molecules by inducing rotational excitation. This is the correct principle. So this is correct.

• C: giving vibrational energy to water molecules
  Vibrational transitions are typically associated with infrared radiation, not microwaves. So this is incorrect.

• D: giving translational energy to water molecules
  Translational kinetic energy increases as a consequence of heating, but the direct microwave interaction is not translational excitation. So this is incorrect.

Final answer

Therefore, the microwave oven acts on the principle of $\boxed{\text{giving rotational energy to water molecules}}.$