Step 1: Sentence Comprehension
The sentence indicates extreme fatigue, impeding the ability to keep eyes open. An adverb signifying "nearly not" or "with significant effort" is required.
Step 2: Option Evaluation
- Hardly: An adverb denoting "almost not" or "scarcely."
Example: “She was so tired that she could hardly keep her eyes open.”
This conveys she could just manage to keep her eyes open, which aligns with the meaning.
- Hard: An adverb meaning "with effort" or an adjective.
Example: “She was so tired that she could hard keep her eyes open.”
Grammatically incorrect; "hard" does not modify "could" suitably in this context.
- Barely: An adverb meaning "only just" or "almost not."
Example: “She was so tired that she could barely keep her eyes open.”
This is grammatically correct and semantically similar to "hardly." However, "hardly" is more idiomatic here.
- Clearly: An adverb meaning "in a clear manner."
Example: “She was so tired that she could clearly keep her eyes open.”
This suggests she could effortlessly keep her eyes open, contradicting the intended meaning.
Step 3: Determination
Both "hardly" and "barely" are suitable.