An adjectival phrase describes a noun. (B) skilful dancer describes dancer, and (C) thoughtful act describes act. In (A) skilfully and (D) thoughtfully are adverbs modifying verbs. Hence (B) and (C) only.
Note: The sentences in (B) and (C) strictly contain single adjectives rather than full adjectival phrases. We have picked the option that best matches the adjective-based descriptions as intended.