Q--Could a white belt defeat Jon Jones in a Brazilian jiu-jitsu match?
A--My step-by-step reasoning: A white belt is the lowest ranking in Brazilian jiu-jitsu. Jon Jones has a purple belt in Brazilian jiu-jitsu under Roberto Alencar. A purple belt is the second highest ranking in Brazilian jiu-jitsu. Jon Jones is one of the greatest combat sports athletes to ever live.
So, the answer is no.

Q--Was the man who played the male lead in Mrs. Doubtfire known for his humour?
A--My step-by-step reasoning: Robin Williams played the male lead in Mrs. Doubtfire. Robin Williams had a prolific standup and film comedy career.
So, the answer is yes.

Q--Are the knights in the Medieval Times show not authentic knights?
A--My step-by-step reasoning: The Medieval Times show is popular in the United States. The United States does not actually bestow knighthood on its citizens. Authentic knights are citizens of certain countries, mainly the United Kingdom, who are given the honorary title by their country.
So, the answer is yes.

Q--Could a llama birth twice during War in Vietnam (1945-46)?
A--
My step-by-step reasoning: The War in Vietnam (1945-46) lasted around 6 months. The gestation period for a llama is 11 months.
So, the answer is no.