The correct answer is: more.
The word “more” means “greater in amount, number, or degree.” In this case, the speaker is saying that their monthly income is greater than 1,000,000/- Rs.
The word “down” means “moving or directed towards a lower level or position.” In this case, the speaker is not saying that their monthly income is moving or directed towards a lower level or position.
The word “below” means “lower in position or rank than something else.” In this case, the speaker is not saying that their monthly income is lower in position or rank than 1,000,000/- Rs.
The word “under” means “lower than or covered by something.” In this case, the speaker is not saying that their monthly income is lower than or covered by 1,000,000/- Rs.