How do you write a chemical equation to represent “Gaseous ammonia reacts with gaseous oxygen to form gaseous nitrogen monoxide and gaseous water”?

The reaction described in the question has two reactants and two products. Ammonia contains the element nitrogen, while nitrogen and oxygen make up nitrogen monoxide.

This statement comprises two reactants and two products: "Gaseous ammonia combines with gaseous oxygen to generate gaseous nitrogen monoxide and gaseous water." Ammonia and oxygen are the reactants, whereas nitrogen monoxide and water are the products.

A chemical equation is defined as the representation of the reaction that occurs when two or more substances react together to form several new products. 

The formula for the compound ammonia is NH₃, which has two elements, nitrogen and hydrogen. The formula for the compound oxygen, which contains just one element, oxygen, and two oxygen atoms, is O₂. Since nitrogen and oxygen are two of the components that make up the molecule nitrogen monoxide, its chemical formula is NO. Since water is made up of two elements, hydrogen and oxygen, its chemical formula is H₂O.
We can thus formulate the equation as follows by include all the chemicals as the reaction:

${\mathrm{NH}}_3+{\mathrm{O}}_2\to \mathrm{NO}+{\mathrm{H}}_2\mathrm{O}$

Since every component in the statement is in a gaseous form, we must include the symbol (g) with each compound in the equation. Consequently, the formula will be

${\mathrm{NH}}_3\left(g\right)+{\mathrm{O}}_2\left(g\right)\to \mathrm{NO}\left(g\right)+{\mathrm{H}}_2\mathrm{O}\left(g\right)$

The equation for gaseous ammonia reacts with gaseous oxygen to give gaseous nitrogen monoxide and gaseous water is:

$4{\mathrm{NH}}_3 \left(\mathrm{g}\right)+5{\mathrm{O}}_{2 }\left(\mathrm{g}\right)\to 4\mathrm{NO} \left(\mathrm{g}\right)+6{\mathrm{H}}_2\mathrm{O} \left(\mathrm{g}\right) .$

Hence, option A is correct.