The reaction between nitrogen and hydrogen to give ammonia is an example of :

As a result of the bond breaking and bond building processes, one mole of ammonia combines with three moles of hydrogen to produce two moles of ammonia. There is only one product being produced. It is a reversible exothermic process. The Haber's process's balanced response is:
${\mathrm{N}}_{2\left(\mathrm{g}\right)}+ 3{\mathrm{H}}_{2\left(\mathrm{g}\right)}\to 2{\mathrm{NH}}_{2\left(\mathrm{g}\right)}$

Reactions that decompose a single reactant into two or more compounds are known as decomposition reactions. Simply said, these reactions are the antithesis of chemical synthesis. For instance, calcium carbonate breaks down into calcium oxide and carbon dioxide.
Combination reactions occur when two or more compounds combine to create a single product or when atoms make and break bonds with one another to create new molecules. Combination reactions are the name given to such reactions. For instance, nitrogen and hydrogen are used to create ammonia, whereas carbon dioxide and water are used to create glucose.
Reactions known as "synthesis" are those in which, typically, components combine to generate compounds. As an illustration, consider the synthesis of water with hydrogen and oxygen and the synthesis of sulfur dioxide with sulfur and oxygen.
Exchanges of ions between compounds occur during displacement reactions. For instance, lead chloride is created by combining lead and copper chloride solution with iron and copper sulfate solution.

Hence, the correct answer is option B.