**Core Concept**
The four polypeptide chains (globins) in the hemoglobin molecule are connected by a network of hydrogen bonds and disulfide bridges, forming a complex quaternary structure. This quaternary structure is crucial for the proper functioning of hemoglobin in oxygen transport.

**Why the Correct Answer is Right**
The four polypeptide chains (globins) in the hemoglobin molecule, specifically the alpha and beta chains, are joined together by a type of covalent bond called a disulfide bridge. This disulfide bridge is formed between the cysteine residues of the globin chains, specifically between the Cys-93 of one chain and the Cys-94 of another adjacent chain. This disulfide bridge helps to stabilize the quaternary structure of hemoglobin and maintains its functional properties.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the type of bond that joins the globin chains together.
**Option B:** This option is incorrect because it does not accurately describe the structure that connects the globin chains.
**Option C:** This option is incorrect because it does not provide a clear description of the connection between the globin chains.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the disulfide bridges in hemoglobin help to maintain its quaternary structure, which is critical for its oxygen-binding properties. This is why mutations that disrupt these disulfide bridges, such as those seen in sickle cell anemia, can lead to significant changes in hemoglobin function.

**Correct Answer: D. Disulfide bridges.**