Read the sentence carefully and choose suitable preposition for the purpose.…

Solution:

The correct preposition is on.

• Use "on" after "congratulations" when referring to an achievement: "congratulations on your success."

• Prefer the plural "congratulations" in this phrase. The singular "congratulation" is uncommon in this context.

• Other prepositions:

  • "at" is not used here.

  • "upon" is rare or formal; "on" is the natural choice.

  • "for" indicates purpose in other constructions but is not standard with "congratulations" to mark an achievement.

Corrected sentence:

We offer the heartiest congratulations on your success.

Note: You can also say "congratulations to you" when directly addressing the person, but to mention the achievement itself, use "on."