Final answer:
The invaginations of the sarcolemma that extend into a muscle cell, facilitating the conduction of action potentials, are called T-tubules.
Step-by-step explanation:
The invaginations of the sarcolemma penetrating deep into the interior of the muscle cell are called T-tubules (option C). The T-tubules are crucial for conducting the action potential into the interior of the muscle cell, allowing for the initiation of muscle contraction. The sarcoplasmic reticulum (SR) and T-tubules form a triad structure that surrounds the myofibrils, the cylindrical structures containing the contractile units known as sarcomeres. When the action potential travels down the T-tubules, it triggers the release of calcium from the adjacent SR into the sarcoplasm, thereby initiating the contraction of muscle fibers.