Answer:
The incorrect one is: La silla son roja.
Step-by-step explanation:
a. This exercise is about the grammatical concordance. In our example, it means that the verb and the adjective have to agree with the subject.
SUBJECT: la silla (feminine and singular)
If it is singular, then the verb has to be singular.
la silla + es
If the noun is feminine and singular, then the adjective has to be feminine singular.
la silla + roja
La silla es roja. CORRECT
b. Las sillas son rojas. CORRECT
las sillas (the subject) is feminine and plural
son is plural
rojas is feminine and plural
c. El escritorio es amarillo.
el escritorio (the subject) is masculine and singular
es is singular
amarillo is masculine and singular
*La silla son roja. INCORRECT
la silla and roja is singular feminine and son is plural