The correct answer is: If.
The word “if” is used to introduce a condition or supposition. In this case, the speaker is imagining a condition in which they are the richest person in the world.
The word “when” is used to introduce a time or event. In this case, the speaker is not imagining a specific time or event, but rather a general condition.
The word “unless” is used to introduce a condition that must be met in order for something to happen. In this case, the speaker is not saying that there is a condition that must be met in order for them to be the richest person in the world.
The word “not any” is used to deny the existence of something. In this case, the speaker is not denying the existence of a condition that would make them the richest person in the world.
In conclusion, the correct answer is “if” because it is the only word that introduces a condition or supposition.