Answer:
Among these elements the strontium (Sr) has lowest first ionization energy.
Step-by-step explanation:
The first ionization energy is the energy required to remove one mole of the most loosely held electrons from one mole of gaseous atoms to produce 1 mole of gaseous ions each with a charge of 1+.
The first of two main methods which scientists use to calculate the ionization energy is the Subtraction Method.