Final answer:
The posterior cricoarytenoid muscle shortens the vocal cords, the triceps brachii extends the forearm, and the gastrocnemius is the largest muscle in the lower leg.
Step-by-step explanation:
The muscle that shortens the vocal cords, which could be associated with relaxing them, is c) the posterior cricoarytenoid muscle. This muscle plays a critical role in voice modulation by controlling the tension of the vocal cords. Contrary to this, muscles that add tension to the vocal cords, thereby raising the pitch, are the cricothyroid muscles. When it comes to the question about which muscle extends the forearm, the correct answer is b) triceps brachii. It's important to distinguish this action from others, like the biceps brachii, which is primarily responsible for flexing the forearm.
As per the question about the largest muscle in the lower leg, the correct answer is b) gastrocnemius. This muscle, along with the soleus, forms the calf muscle and is pivotal for walking, running, and jumping, as it helps to plantarflex the foot at the ankle joint. Additionally, keeping in mind that during flexion, the angle between bones is decreased, we understand that muscles play precise roles in body movements, often working in pairs to produce opposite actions.