**Core Concept**
The electron transport chain (ETC) is a crucial process in cellular respiration, where electrons are passed through a series of protein complexes to generate ATP. FADH2 is an electron carrier that donates electrons to the ETC, but it does so at a different point than NADH.

**Why the Correct Answer is Right**
FADH2 enters the ETC via Complex II, also known as Succinate Dehydrogenase (SDH). This complex is unique in that it is both part of the ETC and the citric acid cycle. FADH2 is generated in the citric acid cycle when Succinate is converted to Fumarate, and it then donates its electrons to the ETC at Complex II. This results in a smaller proton gradient and less ATP generated per electron pair compared to NADH entering at Complex I.

**Why Each Wrong Option is Incorrect**
**Option A:** There is no Complex Ia in the ETC. NADH enters the ETC via Complex I, which is the first complex in the chain.
**Option B:** Complex III, also known as Cytochrome b-c1 complex, is not where FADH2 enters the ETC.
**Option C:** Complex IV, also known as Cytochrome oxidase, is the final complex in the ETC and is involved in the transfer of electrons to oxygen, not the entry of FADH2.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the electron transport chain is a critical process in cellular respiration, and the entry points of different electron carriers (like NADH and FADH2) determine the amount of ATP generated per electron pair.

**Correct Answer:** C. Complex II.