The citric acid cycle, also known as the Krebs cycle or the TCA cycle, takes place in the mitochondrial matrix.
Explanation: The mitochondrial matrix is the innermost compartment of the mitochondria and is where the enzymes that facilitate the steps of the citric acid cycle are located. This cycle is a key component of cellular respiration, as it plays a crucial role in the breakdown of carbohydrates, fats, and proteins to produce energy. The other options, such as the cytoplasm, stroma, nucleus, and intermembrane space, do not contain the necessary machinery for the citric acid cycle, making the mitochondrial matrix the correct answer.