The pressure at which a substance in its gaseous state can exist in equilibrium with the same substance in its liquid state at a given temperature is

I researched it and I think the answer is vapor pressure 

For a neutral atom, the atomic number is equal to number of protons and electrons. Therefore, this atom has 50 protons and 50 electrons. The mass number is the sum of the number of protons and neutrons. By difference, the number of the neutrons is 75. Thus, the answer is letter A.