Answer :

Photosynthesis is a process used by plants, algae, and some bacteria to convert light energy, typically from the sun, into chemical energy in the form of glucose. This process occurs in chloroplasts within plant cells and involves the transformation of carbon dioxide and water into glucose and oxygen.

The chemical equation for photosynthesis can be represented as:

[tex]\[ 6CO_2 + 6H_2O + light \, energy \rightarrow C_6H_{12}O_6 + 6O_2 \][/tex]

From this equation, we can see that the reactants involved in photosynthesis are carbon dioxide ([tex]\(CO_2\)[/tex]) and water ([tex]\(H_2O\)[/tex]), while the products are glucose ([tex]\(C_6H_{12}O_6\)[/tex]) and oxygen ([tex]\(O_2\)[/tex]).

Among the given options:
- [tex]\(N_2\)[/tex] (nitrogen gas) is not a reactant in photosynthesis.
- [tex]\(CO_2\)[/tex] (carbon dioxide) is a reactant in photosynthesis.
- [tex]\(O_2\)[/tex] (oxygen gas) is a product, not a reactant, in photosynthesis.
- [tex]\(N_2O\)[/tex] (nitrous oxide) is not involved in photosynthesis.

Therefore, the correct answer to the question "Which gas is a reactant in photosynthesis?" is:

B. [tex]\(CO_2\)[/tex]