Which atom in the ground state requires the least amount of energy to remove its valence electron? (1) lithium atom (2) potassium atom (3) rubidium atom (4) sodium atom

The answer is (2) potassium atom. The atom which requires the least amount of energy meas that it is the easiest atom to be reacted. From the periodic table of elements, the element from left to right is from the easiest reaction to hardest. From up to down is from the hardest to easiest.