**Core Concept**
Protein synthesis is a complex process involving multiple cellular components, including ribosomes, transfer RNA (tRNA), messenger RNA (mRNA), and amino acids. The process involves the translation of genetic information from mRNA into a polypeptide chain.

**Why the Correct Answer is Right**
The correct answer is related to the structural component of the protein synthesis machinery. Ribosomes are the cellular organelles responsible for translating mRNA into a polypeptide chain. They consist of two subunits, the large subunit and the small subunit, which work together to read the mRNA sequence and assemble amino acids into a polypeptide chain. The ribosomes are the site of translation, where the genetic information encoded in the mRNA is converted into a specific sequence of amino acids.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a cellular component involved in protein synthesis. The option is vague and does not provide any relevant information about the process of protein synthesis.
**Option B:** This option is incorrect because it is a type of RNA involved in protein synthesis, but it is not the cellular component responsible for the actual synthesis of proteins.
**Option C:** This option is incorrect because it is a type of molecule involved in protein synthesis, but it is not the cellular component responsible for the synthesis of proteins.

**Clinical Pearl / High-Yield Fact**
The process of protein synthesis is essential for the production of all proteins in the body, including enzymes, hormones, and structural proteins. Understanding the cellular components involved in protein synthesis is crucial for understanding various diseases, such as genetic disorders, where mutations in genes can affect protein synthesis.

**Correct Answer: D. Ribosomes**