(c) Neuromuscular junction
At the end of the axon, the electrical impulse sets off the release of chemicals that cross the synapse — this junction between a neuron and a muscle cell is the neuromuscular junction.
Source: Chapter 6, Section 6.1
The passage states: "At the end of the axon, the electrical impulse sets off the release of some chemicals… A similar synapse finally allows delivery of such impulses from neurons to other cells, such as muscle cells" — this site is the neuromuscular junction (shown in Fig. 6.1b). Students often confuse "synapse" with "neuromuscular junction"; remember that a neuromuscular junction is the specific synapse between a motor neuron and a muscle fibre where electrical → chemical conversion enables muscle action.