D. The total number of gas molecules will decrease.
Explanation:
According to Le Chatelier's principle, an increase in pressure in a system involving gases will cause the total number of gas molecules to decrease.
Le Chatelier's principle states that "if any conditions of a system in equilibrium is changed, the system will adjust itself in order to annul the effect of the change".
In a reaction involving gases, a change in pressure affects only reactions in a gaseous phase.
An increase in pressure on an equilibrium system will shift the position of the equilibrium to the side having smaller volume and vice versa.
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