Margaret Thatcher became the first female Prime Minister of the United Kingdom in 1979. She served as Prime Minister for 11 years, making her the longest-serving British Prime Minister of the 20th century. She was a controversial figure, but she is also considered to be one of the most influential British Prime Ministers of all time.
Option A is incorrect because Margaret Thatcher became Prime Minister in 1979, not 1998.
Option B is incorrect because Margaret Thatcher became Prime Minister in 1979, not 1989.
Option C is the correct answer. Margaret Thatcher became Prime Minister in 1979.
Option D is incorrect because Margaret Thatcher became Prime Minister in 1979, not 1800.