**Core Concept**
The Electron Transport Chain (ETC) is a series of protein complexes located in the mitochondrial inner membrane, playing a crucial role in cellular respiration by generating ATP through the transfer of electrons. The ETC consists of several complexes, each with a specific function in the electron transport process.

**Why the Correct Answer is Right**
Complex IV, also known as Cytochrome c Oxidase, is the final complex in the ETC and is responsible for transferring electrons to oxygen (O2), resulting in the formation of water (H2O). This complex contains **cytochromes** and **copper centers** that facilitate the electron transfer to O2.

**Why Each Wrong Option is Incorrect**
**Option A:** NADH Dehydrogenase (Complex I) transfers electrons from NADH to Coenzyme Q, but does not react directly with O2.
**Option B:** Succinate Dehydrogenase (Complex II) transfers electrons from succinate to Coenzyme Q, but also does not react directly with O2.
**Option C:** Cytochrome b-c1 Complex (Complex III) transfers electrons from Coenzyme Q to Cytochrome c, but does not react directly with O2.

**Clinical Pearl / High-Yield Fact**
Remember that Complex IV is the only complex in the ETC that directly reacts with O2, making it a critical step in the electron transport process and ATP production.

**Correct Answer:** D. Complex IV (Cytochrome c Oxidase)