Final answer:
Strontium (Sr) has the second smallest ionization energy among the given options.
Step-by-step explanation:
The element with the second smallest ionization energy among the given options is strontium (Sr).
Ionization energy is the energy required to remove an electron from an atom or ion in the gaseous state. Elements with a smaller ionization energy require less energy to remove an electron, indicating that they are more likely to form positive ions.
From the options provided, strontium (Sr) has the second smallest ionization energy after potassium (K), making it the correct answer. Cesium (Cs) and barium (Ba) have larger ionization energies.