Fill in the blanks with the most appropriate option:-

Rahul said that he must congratulate Monica …… her excellent exam results.

on

The correct option to fill in the blank is on.

The word "on" is used to indicate a point in time or an occasion. In this context, Rahul is saying that he must congratulate Monica on her excellent exam results.

The other options are not as appropriate:

• For can be used to indicate a reason or purpose. It would not be accurate to use "for" in this context, as Rahul is not congratulating Monica for something that she did.
• To can be used to indicate movement towards something. It would not be accurate to use "to" in this context, as Rahul is not congratulating Monica by moving towards her.
• Of can be used to indicate possession or part of something. It would not be accurate to use "of" in this context, as Rahul is not congratulating Monica of something that she has.