Since after the blank comes an infinitive verb, which is nadadr, then the conjugation of the verb gustar must match the singular form in the simple present. That's why we choose gusta instead of gustan.
A ellas no les faltan cuadros para la exposición
Since after the blank comes a plural noun, which is cuadros, then the conjugation of the verb faltar must match the plural form in the simple present. So we choose faltan rather than falta.