Answer:
The incorrect word is "tells", as highlighted below:
They complain to teachers about Jake and tells on him.
Step-by-step explanation:
The sentences are all correct excerpt for the last one. The problem concerns subject-verb agreement, which means the subject and the verb of the sentence are not agreeing in number. The subject is "they", which is plural, however the verb form "tells" is singular, more specifically used only for the third-person singular. To correct it, all we need to do is remove the final -s:
They complain to teachers about Jake and tell on him.
"Tell" can be used for the all persons of speech in the present tense (I, you, we, and they) with the exception of the third-person singular.