Answer: Prefiero la música de Juanes; él es el mejor cantante porque sus canciones son hermosísimas
Explanation: We must complete the sentence:
Prefiero la música de Juanes; él es ________(the best-2 palabras) cantante porque sus canciones son ________(extremely beautiful).
With comparative (a comparation) or superlative (a cuality in a big degree)
The first blank is completed with something that means "The best", this is "El Mejor" this is a comparative, because its being compared with other singers.
And the second blank must be completed with "Extemely beautiful" this may be "Hermosísimas" which is the superllative of "hermosas"