In the following sentence completion question, choose the correct preposition to fill in the blank. The director congratulated Mr. Varma _____ his success.

Introduction / Context:
This sentence completion question checks your knowledge of correct prepositions used after certain verbs in English. Collocations, or typical word combinations, are very important for sounding natural and for scoring well in competitive exams. The sentence given is: “The director congratulated Mr. Varma _____ his success.” You must choose the preposition that correctly completes this expression.

Given Data / Assumptions:

• Main verb: “congratulated”.
• Object: “Mr. Varma”.
• Phrase to express the reason: “_____ his success”.
• Options: on, for, at, about, over.
• We need the standard English collocation with “congratulate”.

Concept / Approach:
In standard English usage, we say “congratulate someone on something”. For example, “congratulate her on her promotion”, “congratulate them on their victory”. The preposition “on” introduces the specific achievement or occasion for which congratulations are being offered. While “for” might sound acceptable in some informal contexts, exam-oriented English prefers the fixed combination “congratulate on”. Therefore, the correct preposition here is “on”.

Step-by-Step Solution:
Step 1: Identify the verb and the prepositional phrase: “congratulated … his success”. Step 2: Recall the common pattern: “congratulate someone on something”. Step 3: Fit this pattern into the sentence: “congratulated Mr. Varma on his success”. Step 4: Compare this with other options: “for his success”, “at his success”, “about his success”, and “over his success”. Step 5: Recognize that “on his success” is the most natural and grammatically standard collocation.

Verification / Alternative check:
Check other reliable examples: “We congratulated the team on their performance”, “She congratulated him on passing the examination.” These sentences confirm that “on” is the correct preposition after “congratulate” to introduce the achievement. Substituting “for” or “about” in those sentences makes them sound less natural and not exam standard. Hence, “on” is verified as the right choice.

Why Other Options Are Wrong:
“For” is sometimes used colloquially, but the standard collocation tested in exams is “congratulate on”, not “congratulate for”.
“At” is used in expressions like “laugh at” or “good at”, and it does not collocate correctly with “congratulate” in this sense.
“About” is used with “talk about”, “worry about”, etc., but not typically with “congratulate” to indicate reason.
“Over” can sometimes show emotion (“rejoice over”), but “congratulate over his success” is not the standard pattern.

Common Pitfalls:
Students often choose prepositions based on literal meaning or translation from their mother tongue instead of memorizing common English patterns. Another mistake is assuming that “for” is always correct to show a reason, when in many collocations, “on” is preferred, as in “compliment on”, “remark on”, and “congratulate on”. Regularly noting such combinations while reading newspapers and books is a powerful way to avoid such errors.

Final Answer:
The correct preposition is “on”, so the sentence should read: “The director congratulated Mr. Varma on his success.”