Questions: The contractile units of skeletal muscles are - mitochondria - T tubules - microtubules - myofibrils

The answer is the fourth one: myofibrils.

Explanation for each option:

1. Mitochondria: These are the powerhouses of the cell, responsible for producing energy (ATP) through cellular respiration. They are not involved in muscle contraction.
2. T tubules: These are extensions of the cell membrane that penetrate into the center of skeletal and cardiac muscle cells. They help transmit the action potential into the muscle fiber but are not the contractile units themselves.
3. Microtubules: These are components of the cytoskeleton that help maintain cell shape, enable some cell motility, and are involved in intracellular transport. They are not involved in muscle contraction.
4. Myofibrils: These are the contractile units of skeletal muscles. They are composed of repeating units called sarcomeres, which contain the actin and myosin filaments responsible for muscle contraction.

Summary: The contractile units of skeletal muscles are myofibrils.