Before using metal items in a microwave oven, you should check the

A.) Recipe.
B.) Warranty.
C.) Cookware's use and care guide.
D.) Oven's use and care guide.

C) Cookware's use and care guide. Always.

Explanation:

Oven's use and care guide.

Explanation: The microwave oven is a chamber used for baking or heating that uses microwave energy to heat food or other items placed within it.

As with any equipment or appliance, a microwave comes with a use and guide care that instruct users on the use of the equipment and guide on how to use the equipment.

Of all the options listed above the most likely one to find instructions on whether to use a metal items is in the microwave use and guide care.