Answer:
Explanation: ''La silla'' is singular, ''son'' is plural, ''roja'' is singular. <-- there is incoherence in the sentence, therefore it is false.
In the other sentences:
- ''El escritorio es amarrillo'' <-- the full sentence is singular. (correct)
- ''Las sillas son rojas'' <-- the full sentence is plural. (correct)
- ''La silla es roja'' <-- the full sentence is singular. (correct)