Marian had plenty of work to finish
In the correction form, it says that the function of to finish is adverbial.
But why it is not an adjective? Which work? The work to finish. So it is an adjective!!
You’re right in your analysis, but not exactly correct in the terminology.
The phrase is not an adjective, but because it modifies a noun, it is adjectival.
It has no function like an adverb, so it’s not an adverbial, and the answer key is wrong.