The gas given off as a product of photosynthesis is oxygen (O2).

Here's why:

* Photosynthesis Equation: The basic equation for photosynthesis is:

* 6CO2 + 6H2O + light energy → C6H12O6 + 6O2

* This means that carbon dioxide (CO2) and water (H2O) are used in the presence of light energy to produce glucose (C6H12O6) and oxygen (O2).

* Oxygen as a Byproduct: Plants use the energy from sunlight to split water molecules. This process releases oxygen as a byproduct. The oxygen is then released into the atmosphere.

So, the oxygen we breathe is a direct result of photosynthesis!