Step 1: Understand the sentence The sentence implies extreme tiredness, making it difficult to keep eyes open. We need an adverb that conveys “almost not” or “with great difficulty.”
Step 2: Analyze each option - Hardly: An adverb meaning “almost not” or “scarcely.” Sentence: “She was so tired that she could hardly keep her eyes open.” This means she could barely manage to keep her eyes open, which fits perfectly. - Hard: An adverb meaning “with effort” or an adjective. Sentence: “She was so tired that she could hard keep her eyes open.” Grammatically incorrect as “hard” doesn’t modify “could” appropriately here. - Barely: An adverb meaning “only just” or “almost not.” Sentence: “She was so tired that she could barely keep her eyes open.” This is grammatically correct and similar to “hardly.” However, “hardly” is more common in this context - Clearly: An adverb meaning “in a clear manner.” Sentence: “She was so tired that she could clearly keep her eyes open.” This implies she could easily keep her eyes open, which contradicts the meaning.