Final answer:
The correct form of gustar to fill in the blank is 'gustan'.
Step-by-step explanation:
The correct form of gustar to fill in the blank in the sentence 'A ellos les ______ las clases de español' is gustan.
The verb gustar is used to express likes or dislikes. In this case, since the subject is 'las clases de español' (Spanish classes), which is plural, we use the plural form gustan.
For example, we can say 'A ellos les gustan las clases de español' meaning 'They like Spanish classes'.
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