Of those five choices, I think the best would be "Conflict between the people and the French monarchy sparked revolution" because only this one and the other choices "The Jacobins sought to overturn the French monarchy" and "The cry 'Let them eat cake!' initiated the Revolution" involve the expression of some kind of cause. But "The Jacobins sought to overturn the French monarchy" is more specific than "Conflict between the people and the French monarchy sparked revolution" because the Jacobins were only one of the factions seeking to influence change during the French Revolution, and thus if this were chosen as your thesis statement it would not be effective since you would be expected to restrict your essay to just the role of the Jacobins in the revolution. And "The cry 'Let them eat cake!' initiated the Revolution" is even more specific and thus even more inappropriate for a general essay on the causes of the French Revolution, as well as being somewhat inaccurate; "Let them eat cake" was supposedly a statement uttered by Queen Marie Antoinette that was taken to sum up the indifferent attitude of the monarchy and the nobility toward the French people, but almost certainly that alone did not "initiate" the revolution (though it may have been turned into a rallying cry for the anti-monarchy movement, I'm not sure, though still definitely not a cause of the revolution).
And the other two choices, "The guillotine was an effective instrument for instilling fear and conflict" and "The French Revolution started on Bastille Day, July 14, 1789," have nothing to do with the causes of the French Revolution, though the one about the guillotine arguably expresses one of the reasons for its success--and the eventual counterreaction to the revolution that swung France back toward a more traditional European government and society.
