Why is he very tired today?

The statement that he stayed up very late last night logically connects to the question of why he is very tired today. Staying up late disrupts the body's natural sleep cycle and limits the amount of rest one receives. This lack of adequate sleep can lead to fatigue and a decreased ability to function effectively the following day.

The other options could contribute to feelings of tiredness. For example, not sleeping well or waking up too early may also result in fatigue, but they don’t directly indicate a conscious decision that might have caused the tiredness, like choosing to stay up late. Meanwhile, visiting friends all night can suggest social activity but doesn’t necessarily imply he was awake or not resting, making it less directly connected to immediate tiredness compared to staying up late.