In which of the following sentence have the punctuation marks been used properly?

Correct answer: D. None of the three sentences punctuates properly. Each joins two independent clauses with only a comma, which is a comma splice.

• A: "The blue dress was warmer" and "the purple one was prettier" are both complete sentences, so a comma alone cannot join them. The transitional phrase "on the other hand" also lacks the commas that should set it off.
• B: The comma before "on the other hand" does not fix the splice. The correct join is a semicolon: "The blue dress was warmer; on the other hand, the purple one was prettier."
• C: The parenthetical is punctuated correctly, but the comma between the two clauses is still a splice. The correct form is "The blue dress was warmer; the purple one, on the other hand, was prettier."

Core concept: independent clauses are joined by a comma with a coordinating conjunction, or by a semicolon. Conjunctive adverbials such as "however" or "on the other hand" take a semicolon before them when they link two clauses. This sits in the Prelims English-language section and carries over into Mains essay and precis writing, where clean punctuation matters.

Trap: Aspirants see the commas around "on the other hand" in B and C and assume they are correct, without checking whether the two halves are independent clauses that need a semicolon.