Final answer:
The competitive antagonist of acetylcholine receptors at the neuromuscular junction is Tubocurarine, which causes muscle relaxation and paralysis by blocking acetylcholine.
Step-by-step explanation:
The competitive antagonist of the acetylcholine receptors at the skeletal neuromuscular junction is D) Tubocurarine.
d-Tubocurarine blocks competitively the transmitter action of acetylcholine at the neuromuscular junction. This leads to muscle cell insensitivity to motor-nerve impulses and to directly applied acetylcholine, which causes muscle relaxation and paralysis. The muscle fiber retains its normal sensitivity to potassium ions and responds to direct electrical stimulation, indicating that the block is specific to the acetylcholine receptor.