Which would make an effective thesis for an essay on the cause of the french revolution?
conflict between the people and the french monarchy sparked revolution.
the guillotine was an effective instrument for instilling fear and conflict.
the jacobins sought to overturn the french monarchy.
the french revolution started on bastille day, july 14, 1789.
the cry “let them eat cake! ” initiated the revolution.

The best option from the list would be "The guillotine was an effective instrument for instilling fear and conflict," since this is an opinion that needs to be supported with historical evidence. The other options are simply facts (or non-facts). 

Conflict between the people and the French monarchy sparked revolution.

The prompt asks for a cause to the revolution. This statement takes a position and offers two causes to the revolution. 

A thesis must take a position and answer the prompt. A thesis also must be arguable. In the case of a causation prompt, this statement provides two causes which one could argue there are many more than two. By choosing these two, the writer is suggesting they believe these two causes are the most important. 

The answer is