Which statement requires the inclusion of a comma between two coordinate adjectives?

Responses
A He seemed fully conscious to us.He seemed fully conscious to us.
B The fearsome unstoppable dog barked at us viciously.The fearsome unstoppable dog barked at us viciously.
C A curiously large package sat atop our doorstep.A curiously large package sat atop our doorstep.
D The finale of the exciting playoff game left us breathless.

A comma is typically used between two coordinate adjectives—adjectives that are equal in status and that modify a noun in a similar manner. One way to test if adjectives are coordinate is to see if you can insert "and" between them or if you can reverse their order without changing the meaning of the sentence.

Let's apply this to the options provided:

A. "Fully" is an adverb modifying the adjective "conscious," not a coordinate adjective, so no comma is required.
B. "Fearsome" and "unstoppable" could be considered coordinate adjectives, but it's not clear from the sentence whether they are meant to be equal descriptors of "dog." If they are, a comma would be appropriate. Let's see if the tests apply: Can we say "fearsome and unstoppable dog"? Yes. Can we say "unstoppable fearsome dog"? This does not sound as natural, but it still makes sense, which suggests that these could be coordinate adjectives.
C. "Curiously" is an adverb modifying the adjective "large," so it does not require a comma.
D. "Exciting" and "playoff" are not coordinate adjectives because "playoff" is an adjective specifying a type of game, not describing it in the same way that "exciting" does. "Exciting playoff" is a compound modifier for the word "game," and wouldn't typically use a comma.

Based on this analysis, the sentence that requires the inclusion of a comma between two coordinate adjectives, assuming "fearsome" and "unstoppable" are indeed equal in modifying "dog," would be:

B. The fearsome, unstoppable dog barked at us viciously.

However, even for option B, whether these adjectives are truly coordinate could be subject to the writer's intent. Without clear context that establishes both adjectives as equal modifiers, it can be ambiguous. If the writer does not view them as coordinate, no comma would be necessary.