Step 4: Compare the options provided in the question. Carbon dioxide ($CO_2$) is used by plants during photosynthesis, not released. Glucose ($C_6H_{12}O_6$) is a product of photosynthesis but is primarily used by plants themselves. Nitrogen ($N_2$) is not directly involved in photosynthesis. Oxygen ($O_2$), however, is released by plants and is essential for animal survival.