Final answer:
A muscle with two heads is called a Option a) biceps, and the triceps brachii is the muscle responsible for extending the forearm.
Step-by-step explanation:
A muscle with two heads (origins) would be called a option a) biceps.
When looking at the anatomy of muscles, the prefix used in their names can often indicate the number of origins they have. For example, 'bi-' as in biceps brachii indicates that the muscle has two origins, and 'tri-' as in triceps brachii indicates three origins. This nomenclature is important when understanding muscle groups and their functions.
In response to the original question about which muscle extends the forearm, the correct answer would be triceps brachii.