**Core Concept**
Mitochondrial electron transport chain (ETC) is a critical process for generating ATP in cells. FADH2 is a key electron donor that enters the ETC at a specific complex. This process involves the transfer of electrons from FADH2 to oxygen, resulting in the production of ATP.

**Why the Correct Answer is Right**
FADH2 enters the respiratory chain through Complex II (also known as Succinate Dehydrogenase or Succinate:Quinone Oxidoreductase). This complex is unique because it is both a part of the ETC and the citric acid cycle (Krebs cycle). FADH2 is formed during the citric acid cycle and then directly feeds into Complex II, bypassing the primary entry point of NADH into the ETC. This results in a lower energy yield compared to NADH, which enters at Complex I.

**Why Each Wrong Option is Incorrect**
**Option A:** Not applicable, as this is not a valid option.
**Option B:** This is incorrect because Complex I (NADH Dehydrogenase) is the primary entry point for NADH, not FADH2.
**Option C:** This is incorrect because Complex III (Cytochrome b-c1 Complex) is involved in transferring electrons between Complex II and Complex IV, but it is not the entry point for FADH2.

**Clinical Pearl / High-Yield Fact**
Remember that FADH2 bypasses Complex I and enters the ETC at Complex II, resulting in a lower ATP yield compared to NADH. This is a key concept in understanding energy production in cells.

**Correct Answer: C. Complex II (Succinate Dehydrogenase)**