In each of the questions given below, a statement with a highlighted phrase is given. Choose the most appropriate replacement from the given options which could replace the highlighted phrase to make the statements grammatically and contextually correct. If the highlighted phrase is already correct then choose option “No replacement required” as your response.

Living in Mumbai is more costlier than living in Delhi.

The correct answer is is costlier than.

Step-by-step Explanation:

1. Analyze the original sentence:
The given sentence is: "Living in Mumbai is more costlier than living in Delhi."

2. Identify the grammatical error:
The word "costlier" is already in its comparative form (costly → costlier → costliest).
Using "more" along with an already comparative adjective ("costlier") results in a double comparative error, which is grammatically incorrect in standard English.

3. Evaluate the options:
- "is more costlier then": Contains the double comparative error ("more costlier") and misspells "than" as "then".
- "is costlier than": Correctly uses the simple comparative form "costlier" followed by the conjunction "than" used for comparisons.
- "is much costlier than": While grammatically plausible with an intensifier, the most direct and standard correction to fix the double comparative is "is costlier than".
- "are most costly than": Uses a plural verb "are" for a singular subject ("Living in Mumbai") and incorrectly uses superlative "most" with "than".

4. Conclusion:
Replacing "more costlier than" with "is costlier than" makes the sentence grammatically correct: "Living in Mumbai is costlier than living in Delhi."