He stoma is the innermost compartment of a cytoplasm . Here, the Choose... occur(s), breaking down Choose... acid into carbon dioxide in a series of reactions.

The matrix is the innermost compartment of a mitochondrion. Here, the Kreb's cycle occur breaking down pyruvic acid into carbon dioxide in a series of reactions.

Note: The question is incomplete. The complete question is given below:

The (matrix / stoma / cytoplasm / nucleolus) is the innermost compartment of a (cytoplasm / cell wall / mitochondrion / nucleus). Here, (glycolysis / the Krebs cycle / fermentation / NAD+) occur(s), breaking down (citric / pyruvic / lactic / nitric) acid into carbon dioxide in a series of reactions.

Explanation:

The mitochondria is a double-membrane bound cell organelle found in the cytoplasm within which the biochemical processes of cellular respiration and energy production occur. Cristae are folds of inner membrane that project into the matrix. The matrix is a gel-like material in the innermost compartment of a mitochondria and is the site for the Kreb's or Citric cycle.

The Kreb's cycle is the third stage of cellular respiration (the first is glycolysis in which a molecule of glucose is broken down into two molecules of pyruvic acid; the second is the preparatory stage which oxidizes two molecules of pyruvic acid to two molecules of acetyl-CoA). In this cycle, the two molecules of  acetyl-CoA produced in the preparatory stage is broken down into carbon dioxide in a series of eight energy-extracting reactions producing 2 ATP, 6 NADH, 2 FADH2 and releasing 4 CO2.

the sphincter muscles at the end of the digestive tract is the anus.