Choose the most appropriate adverb to complete the sentence:-

Although she had initial doubts about the success of the project, it eventually turned out to be _____ successful.

highly

The correct answer is Option (4) → highly

The context suggests a contrast between initial doubts and the final result. The adverb must modify the adjective "successful" to indicate a great degree of success, overcoming the initial uncertainty.

• Highly means to a great degree; very much. (Highly successful means very successful.)

• Barely, merely, and hardly are all limiting or negative adverbs (meaning scarcely, only, or almost not). If the project was "barely successful," it would reinforce the initial doubts, not provide a strong contrast to them.