Final answer:
An isotonic contraction involves two phases: concentric, where the muscle shortens to lift a load, and eccentric, where the muscle lengthens while lowering a load. The correct answer is 'd) Concentric, isometric, eccentric', although isotonic contractions are primarily composed of just the concentric and eccentric phases.
Step-by-step explanation:
An isotonic contraction consists of two phases: concentric and eccentric. In a concentric contraction, the muscle shortens to move a load, such as when the biceps brachii muscle contracts to lift a hand weight upwards. The muscle fibers shorten, and cross-bridges form as myosin heads pull the actin. In an eccentric contraction, the muscle lengthens while lowering a load in a controlled manner, like when the same hand weight is lowered, and the biceps brachii muscle gradually relaxes as tension diminishes.
So, the correct answer to the student's question 'An isotonic contraction consists of three phases __________.' is 'd) Concentric, isometric, eccentric', with the understanding that the isotonic contraction referred to here actually includes two primary phases: concentric and eccentric, rather than three phases.