Option 1: Starch is not the first stable product of photosynthesis. Starch is a polysaccharide that is formed by the combination of multiple glucose molecules. It is often synthesized in plant cells and acts as a storage molecule for glucose.

Option 2: Sucrose is also not the first stable product of photosynthesis. Sucrose is a disaccharide made up of glucose and fructose. It is a major transport sugar in plants and is involved in the movement of carbohydrates from the leaves to other parts of the plant.

Option 3: Phosphoglyceric acid (PGA) is indeed the first stable product of photosynthesis. PGA is a three-carbon compound that is formed during the Calvin cycle, which is the second stage of photosynthesis. It is further converted into other compounds in order to produce glucose and other carbohydrates.

Option 4: Glucose is not the first stable product of photosynthesis. Glucose is a six-carbon sugar and is one of the final products of the Calvin cycle.

Therefore, the correct answer is option 3 - Phosphoglyceric acid.