Final answer:
The correct word to fill in the blank is 'le', which represents 'to him' or 'to Ricardo'.
Step-by-step explanation:
The correct word to fill in the blank in the sentence is le. The sentence states 'A Ricardo gusta escuchar la radio', which translates to 'Ricardo likes to listen to the radio'. In Spanish, the sentence structure requires the use of an indirect object pronoun to indicate who likes to listen to the radio. The correct pronoun to use in this case is 'le', which represents 'to him' or 'to Ricardo'.
Learn more about Spanish pronouns