When Did Scientist Begin to Understand How the Sun Produces

Multiple Choice

When did scientist begin to understand how the Sun produces all the energy that it does?

A) Most of the process was already understood by the ancient Greeks
B) Most of the process was understood by the end of the 18th century (the year 1800)
C) Most of the process was understood by the end of the 19th century (the year 1900)
D) The process was not well understood until the 1930's
E) You can't fool me; astronomers still don't have a clue about how the Sun produces energy