During photosynthesis, the oxygen in glucose comes from:

Explanation

Carbon dioxide (CO2) is one of the reactants in photosynthesis. It is taken in by the plant from the atmosphere through tiny pores called stomata on the surface of leaves. Carbon dioxide is reduced in the light-independent reactions (Calvin cycle) using the energy and electrons generated in the light-dependent reactions. The carbon atoms from carbon dioxide are used to build glucose (C6H12O6).