Option 1 (barely) -> Would mean "only just successful," which doesn't create a strong contrast with her initial doubts.
Option 2 (merely) -> Would mean "only successful" with a limiting sense, again not contrasting well with the doubts.
Option 3 (hardly) -> Would mean "almost not successful," which would actually confirm her doubts rather than contrast them.
Option 4 (highly) -> Means "very successful," creating a strong contrast with her initial doubts.
Hence, Option 4: highly -> The word "Although" at the beginning signals a contrast between her initial doubts and the actual outcome. "Highly successful" provides the strongest positive contrast, showing the project exceeded expectations despite her skepticism -> correct