Answer:
Cl
Step-by-step explanation:
The element Cl will have the strongest ionization energy from the given choices. Most non-metals have higher ionization energy compared to metals. 
Ionization energy is the energy required to remove the most loosely held electron from the gaseous phase of an atom. 
- As you go from left to right on the periodic table, it increases progressive
- From top to bottom, the ionization energy reduces significantly. 
- The attractive force between the protons in the nucleus and the electrons plays a very important role. 
- In metals, they have very large atomic radius, the attractive force on the outer electrons is very weak. 
- This is not the case in non-metals