The correct answer is B. The boy fell off the bicycle.
The preposition “off” is used to indicate that someone or something has moved away from the top or edge of something. In this case, the boy has moved away from the top of the bicycle.
The preposition “of” is used to indicate that someone or something is a part of something else. In this case, the boy is not a part of the bicycle.
The preposition “from” is used to indicate that someone or something has moved away from a place. In this case, the boy has not moved away from a place.
The preposition “under” is used to indicate that someone or something is located beneath something else. In this case, the boy is not located beneath the bicycle.