Concept:Secondary sources are materials that interpret or analyze primary sources. They are created by authors who did not directly experience the event.
Explanation:A primary source is a firsthand account (e.g., a letter, newspaper clipping from the time, or a photograph).
A letter by Bhagat Singh is a primary source because he wrote it himself.
A newspaper clipping from March 1931 is a primary source because it reports events at that time.
A photograph of Bhagat Singh in jail is a direct visual record, hence a primary source.
A textbook on Indian revolutionaries is a secondary source because it is written by someone who studied and compiled information from various primary and secondary sources, offering analysis and interpretation.
Answer:Option C – A textbook on Indian revolutionaries