From the options provided below, select the most suitable alternative that can replace the bolded phrase in the sentence to ensure grammatical and contextual accuracy. If the sentence is already correct as written, select "No replacement required."

Commuting by taxi is more costlier than commuting by metro.

The correct option is is costlier than.

Explanation:
In English grammar, using double comparatives is incorrect. The word costlier is already the comparative degree of the adjective costly (costly → costlier → costliest). Adding "more" before an already comparative adjective like "costlier" creates redundancy ("more costlier"), which is grammatically incorrect.

Therefore, "more costlier than" must be replaced simply by "is costlier than" to make the sentence grammatically accurate.

Correct Sentence:
"Commuting by taxi is costlier than commuting by metro."