Step 1: Understand the adjective and adverb usage.
The word "completely" is an adverb that modifies the adjective "open," and it fits the context here, meaning the question remains entirely open. Step 2: Analyze each option.
- (A) complete: Incorrect. "Complete" is an adjective and cannot correctly modify "open" in this context.
- (B) to complete: Incorrect. The infinitive form "to complete" is not suitable here.
- (C) completely: Correct. "Completely" is the right adverb to describe the manner in which the question remains open.
- (D) compliitely: Incorrect. "Compliitely" is a misspelling of "completely." Step 3: Conclusion.
The correct sentence is "It remains a completely open question."