Final answer:
The wattage absorbed by 300g of water in a microwave oven remains 900 W, the same as the power output of the microwave.
Step-by-step explanation:
If 900 W of radiation in a microwave oven is absorbed by 300 g of water in a very lightweight cup, the amount of radiation absorbed by the water is still 900 W. The wattage does not change with the mass of the water being heated. Therefore, the correct answer is (c) 900 W.
Microwave ovens produce electromagnetic radiation in the radio portion of the spectrum. The energy absorbed by water molecules causes an increase in their rotational energies, which is transferred as heat to the food. The power level of the microwave determines how quickly the food is heated based on the energy transfer rate, measured in watts.