Multiple choice The first stable compound formed in photosynthesis of C3 plants is phosphoglyceric acid starch pyruvic acid ribulose diphosphate Reveal answer Fill a bubble to check yourself A Correct answer Explanation In C3 photosynthesis, the first stable product of carbon fixation is 3-phosphoglyceric acid (3-PGA). It is formed when carbon dioxide reacts with ribulose-1,5-bisphosphate.