She is ____interested in you.

She is (C) deeply interested in you.
The plain and clear explanation:-
=deeply, adv.,far; profoundly; intensely; to love deeply
is interested in you=predicate of sentence.
is=verb
deeply=adv
eg:-She felt her man's marriage deeply.
eg:-She is deeply interested in her man.
eg:-Hence; She is deeply interested in you.

C. deeply is形容詞後應用副詞.

She is deeply interested in you.
應使用副詞deeply來修飾述部is interested in you，表示程度很高。