**Answer:**

To determine the number of molecules of glycine required to release one molecule of CO2 during photorespiration, we need to understand the process of photorespiration and the reactions involved.

**Photorespiration:**
Photorespiration is a metabolic pathway that occurs in plants, particularly in the presence of high oxygen levels and low carbon dioxide levels. It involves a series of reactions that occur in the chloroplasts, peroxisomes, and mitochondria of plant cells. The main purpose of photorespiration is to salvage carbon from a toxic compound called glycolate, which is formed when the enzyme Rubisco catalyzes the addition of oxygen to ribulose-1,5-bisphosphate (RuBP) instead of carbon dioxide.

**Steps of Photorespiration:**
1. Oxygenation of RuBP: In high oxygen conditions, Rubisco catalyzes the addition of oxygen to RuBP, resulting in the formation of one molecule of 3-phosphoglycerate (3-PGA) and one molecule of phosphoglycolate.
2. Conversion of phosphoglycolate to glycine: The phosphoglycolate formed in the previous step is transported to the peroxisomes, where it is converted into glycine through a series of reactions.
3. Conversion of glycine to serine: The glycine formed in the previous step is transported to the mitochondria, where it is converted into serine.
4. Conversion of serine to glycerate: The serine formed in the previous step is transported back to the peroxisomes, where it is converted into glycerate.
5. Conversion of glycerate to 3-PGA: The glycerate formed in the previous step is transported back to the chloroplasts, where it is converted into 3-PGA, completing the cycle.

**Number of Molecules of Glycine:**
From the steps mentioned above, we can observe that for every molecule of CO2 released during photorespiration, two molecules of glycine are required. This is because the conversion of phosphoglycolate to glycine and the subsequent conversions of glycine to serine and serine to glycerate all involve one molecule of glycine each. Therefore, the correct answer is **option c, 2 molecules of glycine** are required to release one molecule of CO2 during photorespiration.

It is important to note that photorespiration is an energetically costly process for plants as it consumes ATP and reduces the efficiency of carbon fixation. This is one of the reasons why plants have evolved various mechanisms to minimize photorespiration, such as the C4 and CAM pathways, which help reduce the oxygenation of RuBP by concentrating carbon dioxide around Rubisco.