Final answer:
An increase in pressure generally favors the formation of the denser phase of matter, shifting equilibrium towards compounds with fewer moles of gas in accordance with Le Chatelier's principle.
Step-by-step explanation:
The main answer to the question "A(n) ________ in pressure favors the formation of the more dense phase." is (a) Increase. When pressure is increased, it typically drives a phase change towards the phase that has a higher density. In the context of physical chemistry and phase equilibria, Le Chatelier's principle can be applied where an increase in pressure will shift an equilibrium towards the side with fewer moles of gas, which is usually the denser phase, such as a liquid or solid. For example, increasing pressure can convert a gas into a liquid, or a liquid into a solid.