Final answer:
The triceps brachii is the muscle that extends the forearm. It works as an extensor and pairs with the biceps brachii, which is a flexor muscle for bending the elbow.
Step-by-step explanation:
The muscle responsible for extending the forearm is the triceps brachii. This three-headed muscle works as an extensor, which means it helps to open the limb. In contrast to the triceps, the biceps brachii is a flexor muscle that closes the limb by flexing the forearm.
This configuration, where muscles work in pairs, is typical in the human body. For example, the biceps contract to bend the elbow while the triceps relax, and when extending the arm, the triceps contract while the biceps relax. The mechanical advantage provided by muscle and tendon attachment close to joints allows for more significant force to be exerted within the body than what is applied externally through the limbs.