Final answer:
The most likely cause of a single brake light not working is a bad bulb. This is the simplest and first issue to check, and replacing the bulb can quickly solve the problem.
Step-by-step explanation:
The most likely cause of a customer's right rear brake light being out is D) A Bad Bulb. This is typically the first item to check when a single brake light is not functioning, as bulbs are subject to regular wear and tear and can burn out. It's a straightforward process to test this by replacing the bulb with a new one. If the bulb replacement solves the issue, then no further troubleshooting is necessary. If not, then one might consider other possibilities such as B) A Broken wire, C) A short in the right rear tail lamp assembly, or even A) A Bad Brake light switch, although the latter often results in all brake lights failing, not just one.