Oxidation of H2O to O2 is a process that occurs during the process of photosynthesis in plants. In this process, water molecules are split into hydrogen ions (H+) and oxygen gas (O2). This reaction is catalyzed by the enzyme known as photosystem II, which is located in the thylakoid membrane of the chloroplast.

The overall reaction can be represented as follows:

2H2O -> 4H+ + 4e- + O2

The process of oxidation involves the loss of electrons. In this case, water molecules lose electrons and are oxidized to produce oxygen gas. Here's a detailed explanation of each option:

a) H2 and O2: This option suggests that water molecules can be oxidized to form hydrogen gas (H2) and oxygen gas (O2). However, this is incorrect as water molecules are not oxidized to produce hydrogen gas. Instead, water molecules are split into hydrogen ions (H+) and oxygen gas (O2).

b) O2-: This option suggests that water molecules can be oxidized to form superoxide ions (O2-). However, this is incorrect as the oxidation of water does not produce superoxide ions.

c) OH-: This option suggests that water molecules can be oxidized to form hydroxide ions (OH-). However, this is incorrect as the oxidation of water does not produce hydroxide ions.

d) O2: This option suggests that water molecules can be oxidized to form oxygen gas (O2). This is the correct answer as the oxidation of water during photosynthesis leads to the production of oxygen gas.

In summary, the correct answer is option 'D' because water molecules can be oxidized to produce oxygen gas (O2) during the process of photosynthesis.