Final answer:
The correct article to use is 'a,' resulting in the sentence 'Ronaldo is a famous footballer,' because 'a' is used before words that start with a consonant sound.
Step-by-step explanation:
The correct answer to fill in the blank in the sentence "Ronaldo is ________ famous footballer" is "a," making the complete sentence "Ronaldo is a famous footballer." We use "a" before words that start with a consonant sound and "an" before words that start with a vowel sound. The definite article "the" is used when referring to a specific item that is known to the listener. Since Ronaldo is one of many famous footballers, and the sentence doesn't specify that he is the only one, "a" is the suitable article to use here.