The correct answer is option 1: absorption of microwaves by matter.

Microwaves are a form of electromagnetic radiation, similar to radio waves and infrared light. When a microwave oven is turned on, it generates microwaves that are directed towards the food being cooked. These microwaves consist of electric and magnetic fields that oscillate rapidly.

The molecules in the food contain water, fats, and certain other substances that can absorb microwave energy. When the microwaves come into contact with the food, the electric field causes the molecules to vibrate, which in turn generates heat. This heat is what cooks the food in a microwave oven.

Option 2 is incorrect because optical fiber is used to transmit light signals, not microwaves. Option 3 is incorrect because it describes the process of microwave amplification using stimulated emission of radiation, which is the principle behind how lasers work, but not microwave ovens. Option 4 is incorrect because metal reflects and blocks microwaves, rather than transmitting them.

Therefore, the correct answer is option 1: absorption of microwaves by matter.