Final answer:
The triceps brachii are the primary muscles responsible for extending the forearm, not the trapezius or posterior deltoid.
Step-by-step explanation:
The statement that a close grip bench press exercises primarily the trapezius and posterior deltoid is false. The primary muscle worked in a close grip bench press is the triceps brachii, which is responsible for extending the forearm. Other muscles involved include the pectoralis major and the anterior deltoids to a lesser extent.
In response to the question 'Which muscle extends the forearm?' The correct answer is b. triceps brachii. The biceps brachii are responsible for flexing the forearm, the brachialis assists in this flexion, and the deltoid is primarily involved in arm abduction. The trapezius and rhomboids position the pectoral girdle and move the scapula but are not primary movers in the forearm extension.