During photosynthesis, unused oxygen atoms form oxygen gas (o2) which is released as a waste product. how did these oxygen atoms originally enter the plant?

they were part of the atp molecules gathered by the chloroplasts.

they were part of the sugar molecules (c6h12o6) the plant brought in for food.

they were part of the carbon dioxide (co2) molecules that entered through the leaves.

they were part of the water molecules (h2o) pulled in through the roots.

The right answer is D.

The first phase of photosynthesis, called the light phase, requires the illumination of the sheet. This photochemical phase corresponds to the capture of light energy by light-absorbing pigments and its conversion into chemical energy in the form of ATP. In this reaction, the photosensitive pigments ("chlorophyll b", carotene and xanthophylls) absorb and then channel the light energy to the "chlorophyll a" whose electrons are brought to a higher energy potential. In this excited state, the electrons of chlorophyll are derived to a system that extracts and stores their energy for later use in the synthesis of sugars. The result of this electron transfer is the splitting of the water molecules to give hydrogen (H) and oxygen (0) atoms. Oxygen escapes in gaseous form O2 into the atmosphere through open stomata. This whole sequence is done in a fraction of a second.

The reaction is as follows:

H2O ==> 1/2 O2 + 2e- + 2H +

Option D.  They were part of the water molecules (H2O) pulled in through the roots

Explanation:

Photosynthesis defined as the process by which plants make there own food by trapping sunlight and using carbon-dioxide and water to produce glucose as energy.

During photosynthesis oxygen is formed as raw material along with glucose after utilizing carbon-dioxide and water.

Photosynthesis process take place in two stages light-dependent and light-independent process. During the light-dependent reactions,  water molecule releases an electron and oxygen and hydrogen atoms gets free. The free oxygen atom combines with another free oxygen atom and forms diatomic oxygen (O2) that released to atmosphere.

This is how oxygen enters the plants as a part of water molecules. through roots.

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."