## Core Concept
Mitochondria are often referred to as the powerhouses of the cell because they generate most of the cell's supply of adenosine triphosphate (ATP), which is used as a source of chemical energy. They are involved in a range of cellular processes including energy production through oxidative phosphorylation, regulation of cellular metabolism, and apoptosis. The processes occurring within mitochondria include the citric acid cycle (Krebs cycle), fatty acid oxidation, and the electron transport chain.

## Why the Correct Answer is Right
The correct answer, , involves understanding which cellular processes are specifically localized to mitochondria. The citric acid cycle (also known as the Krebs cycle or tricarboxylic acid cycle) occurs in the mitochondrial matrix. Fatty acid oxidation and the electron transport chain also take place in mitochondria, with the latter being embedded in the mitochondrial inner membrane. Glycolysis, however, is the process by which glucose is converted into pyruvate, generating a small amount of ATP and NADH in the process. This process occurs in the cytosol of the cell, not within mitochondria.

## Why Each Wrong Option is Incorrect
- **Option A:** The citric acid cycle (Krebs cycle) indeed occurs in mitochondria, specifically within the mitochondrial matrix. This process is crucial for the production of ATP, NADH, and FADH2.
- **Option B:** Fatty acid oxidation takes place in mitochondria. This process involves the breakdown of fatty acids into acetyl-CoA units, which can then enter the citric acid cycle for energy production.
- **Option C:** The electron transport chain is located in the mitochondrial inner membrane and is essential for generating ATP through the process of chemiosmosis.

## Clinical Pearl / High-Yield Fact
A key point to remember is that while mitochondria are crucial for energy production in the cell, glycolysis—the initial step in glucose breakdown—occurs outside of mitochondria, in the cytosol. This is a fundamental distinction that can help in understanding cellular metabolism and the localization of various biochemical pathways.

## Correct Answer Line
**Correct Answer: D. Glycolysis**