Final answer:
A railroad crossing sign is yellow and black, which is highly visible to attract the attention of motorists and pedestrians for safety near the tracks.
Step-by-step explanation:
A railroad crossing sign is designed to alert drivers and pedestrians to the presence of a railroad track intersection and to be cautious. The correct answer to the student's question is c. yellow and black. A typical railroad crossing sign in the United States is indeed yellow with a black 'X' symbol and the words 'RAILROAD CROSSING' printed on it. This design is intended to be highly visible and universally recognizable to ensure safety near the tracks.