Final answer:
Catherine II, also known as Catherine the Great, was the longest-ruling female ruler of Russia, expanding its territory and reinforcing its position as a major European power.
Step-by-step explanation:
The longest-ruling female ruler of Russia was Catherine II, commonly known as Catherine the Great. She was an enlightened despot whose reign remained authoritarian, although she justified her political power through the idea of a social contract. Her rule stretched from her ascension to the throne after the coup that overthrew her husband Peter III in 1762 until her death in 1796. Through her efforts, the Russian Empire expanded significantly, including territories such as the area now known as Ukraine, the northern Caucasus Mountains, and even Alaska. Catherine's influence and policies positioned Russia as a major European power well into the 19th century.