To solve this question, the user needs to know about different sports and their scoring systems.

The correct answer is: B. Boxing

In boxing, the score is not revealed until the end of the match. The judges score each round based on the number of punches landed, defense, and ring generalship. The scores for each round are added up at the end of the match to determine the winner. Spectators and participants do not know the score or the leader until the end of the match.

Option A, Cricket is incorrect because in cricket, both the spectators and participants are aware of the score and the leader throughout the game.

Option C, Rugby is incorrect as the score is updated regularly, and the participants and spectators can know the score.

Option D, Javelin Throw is not a sport where the score is kept hidden until the end of the contest. It is a field event, where competitors throw a spear-like object known as a javelin, and the one who throws the farthest distance is the winner.

Therefore, the correct answer is B. Boxing.