When looking at a Neuromuscular Junction (NMJ), we know that neurotransmitters (NT) are released from the presynaptic cell and they then bind to the receptors that are located on the postsynaptic cell - this causes the effect of the NT being released.
So we are told that NT are still being released, however they are not having an effect. This would mean that they are probably being blocked by something - in this case, it seems that the neurotoxin is the culprit in the blocking of these receptors.
Therefore, if the NT cannot bind to the receptors on the postsynaptic cell, they are not going to have any effect, no matter how much NT is being released.
So the answer in this case is: The neurotoxin is most likely C) Blocking the receptors on the postsynaptic cell.
