The correct response to "¿Cómo es el muchacho?" (which means "How is the boy?") is:
El muchacho es bajo.
This means "The boy is short," and is the correct form because "bajo" is the singular masculine adjective that agrees with "muchacho."
The other options ("El muchacho es bajos," "El muchacho es bajas," and "El muchacho es baja") are incorrect because they do not agree in gender and number with "muchacho."