**Explanation:**

**C3 Plants:**
C3 plants are a type of plants that undergo the Calvin cycle during photosynthesis. In this cycle, the first stable compound formed is a three-carbon molecule called phosphoglycerate (PGA). C3 plants include many common crops such as wheat, rice, and soybeans.

**Radioactive Carbon in CO2:**
When carbon dioxide (CO2) is radioactively labeled, it means that one or more of the carbon atoms in the molecule have been replaced with a radioactive isotope of carbon, such as carbon-14 (^14C). This labeling allows scientists to track the movement and transformation of carbon within a biological system.

**Product of Radioactive Carbon in C3 Plants:**
In C3 plants, the product in which the radioactive carbon can be found is phosphoglycerate (PGA). This is because during the Calvin cycle, CO2 is fixed by the enzyme RuBisCo (ribulose-1,5-bisphosphate carboxylase/oxygenase) to form an unstable six-carbon compound called RuBP (ribulose-1,5-bisphosphate). This compound then breaks down into two molecules of PGA, each containing three carbon atoms.

**Explanation of Options:**
a) PGAL (phosphoglyceraldehyde): PGAL is an intermediate molecule formed during the Calvin cycle and is a product of PGA. However, it does not directly contain the radioactive carbon from CO2.

b) RuBP (ribulose-1,5-bisphosphate): RuBP is the initial compound that reacts with CO2 during the Calvin cycle, but it is not a direct product of radioactive carbon in CO2.

c) PGA (phosphoglycerate): PGA is the first stable product formed during the Calvin cycle and contains the radioactive carbon from CO2. Therefore, this is the correct answer.

d) PEP (phosphoenolpyruvate): PEP is a molecule involved in the formation of glucose during gluconeogenesis, which is not directly related to the Calvin cycle or the fixation of CO2.

**Conclusion:**
The radioactive carbon from CO2 can be found in the product phosphoglycerate (PGA) in C3 plants. This is the first stable compound formed during the Calvin cycle and contains the radioactive carbon.

Carbon dioxide is fixed to PGA thus radioactivity will be dected in PGA