The vapor pressure of a gas is the pressure necessary to keep it in a(n) __________.

Correct Answer: Option A — liquefied state

Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature. It represents the force required to maintain the substance in a liquefied state; if the pressure drops below this point, the liquid will transition into a gaseous state through evaporation.