The correct option is D. The sentence should read: "It's getting much colder outside." In English, we typically use "much" in combination with comparatives like "colder" to intensify their meaning. The word "colder" is the comparative form of the adjective "cold," and it is used to indicate that the temperature is dropping or that it is less warm than before.
Options A, B, and C don't fit grammatically. "More colder" and "less colder" are incorrect because "colder" already indicates a comparison. You don't need to add "more" or "less." The option "most coldly" is also incorrect because "coldly" is an adverb, and we don't use "most" with adverbs in this way. The correct superlative form of "cold" is "coldest," and the correct superlative form of "coldly" is "most coldly," but neither fits in this sentence.
