The French Revolution depicted in Assassin's Creed Unity began in 1789.
Unity opens in the period surrounding the French Revolution, a major political and social upheaval that transformed France. The game uses events such as the storming of the Bastille and the rise of revolutionary factions as a historical backdrop for Arno Dorian's fictional Assassin narrative.
The Bastille was stormed on 14 July 1789, a date now commemorated as France's national holiday. The revolution continued for years, including the Reign of Terror in 1793 and 1794, so 1789 identifies its beginning rather than the whole period.
The dates in the wrong answers refer to other significant events: 1776 to the American Declaration of Independence, 1793 to the execution of Louis XVI and the Reign of Terror, and 1804 to Napoleon's coronation as emperor.