The correct answer is A. Muscle cells.
Muscle cells are the only cells in the human body that can contract and relax. This is because they contain special proteins called actin and myosin, which slide past each other when the cell is stimulated by a nerve impulse. This sliding motion causes the cell to shorten, which is what we call contraction. When the nerve impulse stops, the actin and myosin proteins slide back into place, and the cell relaxes.
Excretory cells are responsible for removing waste products from the body. They do this by filtering the blood and producing urine. Nervous cells are responsible for sending and receiving messages from the brain and spinal cord. Respiratory cells are responsible for exchanging oxygen and carbon dioxide in the lungs.
All of these cells are important for the body to function properly, but only muscle cells can contract and relax.