During aerobic respiration, each pyruvate molecule is broken down to give ______ molecules of carbon dioxide.

The correct answer is Three.

During aerobic respiration, glucose ($C_6{H}_{12}{O}_6$) is first broken down in the cytoplasm via glycolysis to form two molecules of pyruvate, each containing three carbon atoms ($C_3{H}_4{O}_3$). These pyruvate molecules then enter the mitochondria to undergo aerobic oxidation, which completely breaks them down into carbon dioxide ($C{O}_2$) and water.

The step-by-step breakdown of a single pyruvate molecule occurs as follows:
1. Pyruvate Decarboxylation (Link Reaction): Each 3-carbon pyruvate molecule is oxidatively decarboxylated to form a 2-carbon Acetyl-CoA molecule. During this step, one carbon atom is released as one molecule of $C{O}_2$.
2. Krebs Cycle (Citric Acid Cycle): The 2-carbon Acetyl-CoA enters the Krebs cycle, combining with 4-carbon oxaloacetate to form 6-carbon citric acid. Through a series of redox reactions, two more carbon atoms are released as two separate molecules of $C{O}_2$ (during the conversions of isocitrate to $\alpha$-ketoglutarate, and $\alpha$-ketoglutarate to succinyl-CoA).

Adding the carbon dioxide molecules released from both stages:
$1C{O}_2\left(\text{from link reaction}\right)+2C{O}_2\left(\text{from Krebs cycle}\right)=3C{O}_2\text{molecules}$

Thus, each pyruvate molecule is completely oxidized to yield three molecules of carbon dioxide.