Final answer:
The primary muscle responsible for a movement is the agonist, while the muscle that helps the agonist is called a synergist. For instance, in lifting a cup, the biceps brachii acts as the agonist, with the brachialis serving as a synergist. Option b.
Step-by-step explanation:
Understanding Muscle Function and Movement
The muscle that is primarily responsible for bringing about a particular movement is known as the agonist, or prime mover. For example, during the action of lifting a cup, the biceps brachii serves as the agonist. However, other muscles, such as the brachialis, may assist the agonist and are therefore referred to as synergists. Sometimes a synergist also acts as a fixator, providing stabilization at the muscle's origin to ensure efficient movement. In contrast to these, the antagonist muscle has the opposite action to the agonist, helping to maintain body position and control rapid movements.
To directly answer the question, the muscle that is primarily responsible for bringing about a particular movement is b) Agonist. Additionally, the helper that assists an agonist in its work is known as a a) synergist.