The answer is B. estudiante.
The article preceding it, 'las', is the plural feminine article. 'estudiante' in the sentence is singular only, but it is gender neutral. 'son' is the third person plural conjugation of the verb 'ser'. 'inteligentes' is an adjective that would refer to plural subjects. To fix the sentence, change 'estudiante' to 'estudiantes'.
Está makes sense when u read it outloud
The Spanish word for lazy is perezoso
