The correct option is "got it fixed".

This question tests the use of the causative structure in English.
When we arrange for someone else to do a service for us (in this case, having a mechanic in the garage repair the car), we use the causative structure:
have/get + object + past participle.

Let's break down the sentence structure:
1. Subject + Verb: "he got"
2. Object: "it" (referring to the car)
3. Past Participle: "fixed" (the action done to the car)

Therefore, the complete sentence is: "He did not manage to fix the car himself, so he got it fixed in the garage."