Answer: "lo que"
Explanation: We must complete the sentence:
" _________ me gusta de Marc Anthony es su voz bonita."
With the correct relative pronoun.
Relative pronouns are the ones that are related to something that comes after or before, and introduce a relative sentence.
Here a relative pronoun that works is "Lo que"
Lo que me gusta de Marc Anthony es su voz bonita.
Where the pronoun is relative to Marc Anthony's voice.