**Core Concept**
The process of eukaryotic protein synthesis involves various regulatory steps, including initiation, elongation, and termination. **Insulin** plays a significant role in regulating protein synthesis by influencing key components of the translation machinery. One crucial aspect of this regulation is the **phosphorylation** of specific proteins.

**Why the Correct Answer is Right**
Insulin enhances the phosphorylation of **4E-BP1** (eukaryotic translation initiation factor 4E-binding protein 1), which is a key regulator of protein synthesis. When **4E-BP1** is phosphorylated, it releases **eIF4E** (eukaryotic translation initiation factor 4E), allowing it to initiate translation. This process is mediated by the **mTOR** (mechanistic target of rapamycin) pathway, which is activated by insulin.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is not directly related to the insulin signaling pathway in the context of protein synthesis regulation.
**Option B:** Incorrect as it is not the primary target of insulin-induced phosphorylation in the context of enhancing protein synthesis.
**Option C:** Incorrect because, although related to protein synthesis, it is not the specific target enhanced by insulin through phosphorylation.

**Clinical Pearl / High-Yield Fact**
Insulin's role in enhancing protein synthesis is crucial for understanding its effects on metabolism and growth. Remember, insulin signaling activates the **mTOR** pathway, which in turn regulates protein synthesis by phosphorylating key targets like **4E-BP1**.

**Correct Answer:** D. 4E-BP1.