Select the most appropriate option to fill in the blank.

I congratulated him ______ his promotion.

on

The correct option to fill in the blank is on.

The sentence is about expressing congratulations to someone for their promotion. The preposition "on" is used to express congratulations or good wishes on a happy occasion.

The other options are not as appropriate.

• To is used to express a purpose or destination. In this context, it would not make sense to say "I congratulated him to his promotion."
• Because of is used to express a reason or cause. In this context, it would not make sense to say "I congratulated him because of his promotion."
• Due to is also used to express a reason or cause. However, it is more formal than "on" and it is not as commonly used in this context.