💡 Detailed Explanation
The beneficiary structure '〜てもらう' (te morau) translates to 'to have someone do (something for one's benefit)'. It emphasizes appreciation for an action performed by another person upon request. It connects to the 'te' form of verbs. The actor performing the helpful service is strictly marked by the particle 'に' (e.g. 先生に教えてもらう - to have the teacher explain it to me). In this sentence, it translates perfectly as: "I had my teacher explain the part I didn't understand.". The correct option 'もらった' dynamically matches the structural requirements and logical flow of the statement. The other options do not satisfy the grammatical construction or fail to convey the appropriate semantic nuance.