Which of the following is the best definition for secondary source?

You didn't provide a list of options, so let me just explain the difference between primary sources and secondary sources in studying history.

Primary sources are things that come from persons directly involved in the events or lives being studied. So, let's say World War II is our subject. Any diary entries or letters (like the diary of Anne Frank or a letter by a soldier) would count as primary sources. So would any official proclamations or letters or government documents - - such as the Constitution of France or a declaration of war by US Congress.

Secondary sources are written by historians or others after their study of primary sources. A biography of Napoleon, or a historical article about one of America's founding fathers, or a textbook on world history - - all of those would be secondary sources.