He did not manage to fix the car himself, so he _______ in the garage.

The correct answer is "got it fixed".

This question tests our understanding of the causative structure in English, which is used when we arrange for someone else to do a job or task for us rather than doing it ourselves.

The standard causative structure with the verb get is:
Subject + get (in the appropriate tense) + object + past participle (V3)

Let us break down the sentence structure based on this rule:
1. Subject: "he" (already in the sentence)
2. Causative Verb: "got" (past tense of get, matching the past tense "did not manage" in the first clause)
3. Object: "it" (referring to "the car")
4. Past Participle: "fixed" (the V3 form of the verb fix)

Putting these components together, we get: he got it fixed.
This correctly shows that he did not repair the car himself; instead, he arranged for someone else (the mechanic at the garage) to do it for him.