**Core Concept**
The respiratory chain is a series of protein complexes and electron carrier molecules located in the mitochondrial inner membrane that generate energy in the form of ATP during cellular respiration. Mobile electron carriers are crucial in facilitating the transfer of electrons between these complexes.

**Why the Correct Answer is Right**
Cytochrome b is a mobile electron carrier in the respiratory chain that plays a key role in the transfer of electrons during the process of oxidative phosphorylation. It belongs to the cytochrome b-c1 complex, which is a part of the electron transport chain (ETC). Cytochrome b accepts electrons from the reduced coenzyme Q (ubiquinol) and transfers them to cytochrome c1, facilitating the continuation of the electron transport chain. This process ultimately leads to the generation of a proton gradient, which is used to produce ATP.

**Why Each Wrong Option is Incorrect**
* **Option A:** Ubiquinone (Coenzyme Q) is a mobile electron carrier, but it is not the primary carrier in the cytochrome b-c1 complex.
* **Option B:** FADH2 is a coenzyme involved in the electron transport chain, but it is not a mobile electron carrier in the cytochrome b-c1 complex.
* **Option C:** FeS (Iron-Sulfur clusters) are electron carriers in the respiratory chain, but they are not mobile in the same way as cytochrome b.

**Clinical Pearl / High-Yield Fact**
The electron transport chain is a critical component of cellular respiration, and understanding the role of mobile electron carriers like cytochrome b is essential for appreciating the process of oxidative phosphorylation.

**Correct Answer:**
✓ Correct Answer: D. Cytochrome b