When nitrogen reacts with hydrogen under high temperature and pressure to form ammonia gas. This is an example of:
A. Chemical combination
B. Chemical decomposition
C. Physical change
D. None of the above

Hint: Here, in the given chemical process two compounds are reacting with each other and combine to form only one compound. Now you can easily understand the name of the process.

Complete step by step answer:

The reaction given in the question is forming ammonia gas. This process is known as the Haber process.
The Haber Process - The Haber Process combines nitrogen from the air with hydrogen derived mainly from natural gas (methane) into ammonia. The reaction is reversible and the production of ammonia is exothermic.
$N_{ 2 }\quad +\quad 3H_{ 2 }\quad \rightleftharpoons \quad 2NH_{ 3 }\quad +\quad Heat$

This reaction is a combination reaction.
Combination reaction - A combination reaction, also known as a synthesis reaction in chemistry, is when two or more substances, or reactants, combine with each other to form a new product. The product will always be a compound.

Additional information:
Conditions for Haber process -
Catalyst - The catalyst is actually slightly more complicated than pure iron. It has potassium hydroxide added to it as a promoter - a substance that increases its efficiency.
Pressure - The pressure varies from one manufacturing plant to another but is always high. You can't go far wrong in an exam quoting 200 atmospheres.

Note: We should also know that chemical decomposition, or chemical breakdown, is the process or effect of simplifying a single chemical entity into two or more fragments. Chemical decomposition is usually regarded and defined as the exact opposite of chemical synthesis.