The correct answer is D. No improvement.
The sentence “The robber entered into the old man’s room very quietly” is grammatically correct and conveys the intended meaning clearly. The preposition “into” is used to indicate movement from one place to another, and the adverb “very quietly” describes the manner in which the robber entered the room.
The other options are not as clear or concise. Option A, “entered inside,” is redundant, as the word “inside” is already implied by the preposition “into.” Option B, “entered,” is grammatically correct, but it does not provide any additional information about the manner in which the robber entered the room. Option C, “entered in,” is incorrect, as the preposition “in” is not used to indicate movement from one place to another.
Therefore, the correct answer is D. No improvement.