Final answer:
The appropriate word to fill in the blank is 'le'.
Step-by-step explanation:
The appropriate word to fill in the blank in the sentence is le. The sentence should be written as 'A Ricardo, le gusta escuchar la radio.' In Spanish, the pronoun 'le' is used to indicate that someone likes or enjoys something. In this case, it is used to indicate that Ricardo likes listening to the radio.
Learn more about Spanish pronouns