Answer:
felices
Explanation:
Los chicos felices
Translation:
Happy boys
First we discard "felizes" and "felizos" these words in Spanish do not exist. For the choice of "felices" the amount of the subject is taken into account, in this case to refer to the boys is plural and as in Spanish the adjectives are pluralized, the plural of "feliz" (singular) is "felices"
