Question

The oxygen liberated during photosynthesis by green plants come from

Answer 1

oxygen is librated from water in photosynthesis

Answer 2

$\huge\boxed{\mathsf{Water(H_2O)}}$

When Water is broken down during Photosynthesis, the Hydrogen is used in reducing the Carbon Dioxide$(CO_2)$ and the Oxygen$(O_2)$ is released in the Atmosphere as a by-product.