In the sentence, identify the segment that contains a grammatical error.
Have you decided which university you want to join after you finished school?

Correct option is A

Option (a) contains the grammatical error.
Explanation:
The main clause “Have you decided…” is in present perfect tense. The action “after you finished school” refers to a past event connected to the present decision. Therefore, it should be in present perfect or simple present, not simple past.
Correct sentence:
Have you decided which university you want to join after you finish school?
Grammatical Rule Used:
When the main clause is in present perfect, the subordinate clause talking about an expected or habitual action should be in simple present.
Example:
She will go abroad after she completes her degree.