Highest ionization potential in a period is shown by:
(A) Alkali metals
(B) Transition elements
 (C) Halogens
 (D) Noble gases

Hint: The closer and more tightly bound an electron is to the nucleus, the more difficult it will be to remove and higher will be its ionization potential. The ionization potential or ionization energy decreases from top to bottom in groups and increases from left to right across a period.

Complete step by step solution:
The ionization energy or ionization energy of an atom is the amount of energy that is required to remove an electron from a mole of atoms in the gas phase. In a periodic table, the ionization potential decreases from top to bottom in groups while it increases when crossing from right to left in a period. It can be understood by the image shown below,

So, the groups further to the right of the periodic table would have greater ionization potential because they are more stable and they do not want to donate its electrons to other atoms and also there is the greater number of protons (positively charged) that attracts the electrons (negatively charged) which are present in the outer shell thereby requiring more energy to remove an electron from the shell.
Thus, concluding noble gases to be having the highest ionization potential. This is shown by the noble gases because an electron is to be removed from a completely filled orbital which further breaks the stable electronic configuration ns2np6 requiring a large amount of energy.

Hence, the correct option will be (D).

Note: Technically the noble gases have the largest ionization potential, but since they are special and it is not often seen that electrons would even be removed from these elements as they rarely react with other elements, you would usually get confused with halogens having the largest ionization potential as they are present before the noble gases in a period in the periodic table.