There are specific names used for some groups of the periodic table:
group 0/8 (18) → noble gases
group 1 → alkali metals
group 7 (17) → halogens

Elements in the same group have similar properties since they have the same number of electrons in their outer shell. For groups 1-2 and 3-8 it is the same as the group number. For example, phosphorus has 5 electrons in its outer shell. If you use a periodic table with 1-18 group numbers, you should convert groups 13-18 to 3-8 respectively as the aforementioned rule doesn’t necessarily apply for the groups 3-12.

As you go down a group, the elements get bigger as they gain additional shells of electrons.

Group 1 of the period table is called alkali metals.

alkali metals

Since going down a group the atomic radius increases, we should pick the top 3 halogens; fluorine, chlorine, bromine.

fluorine, chlorine, bromine

As you go down a group, the elements get bigger as they gain an extra shell of electrons. Therefore, radium has the largest atoms out of the 3 elements provided.

radium

For groups 1-2 and 3-8 the number of electrons in the outer shell is the same as the group number. Chlorine is in group 7; thus, it has 7 electrons in its outer shell.

7

Group 7 of the period table is called halogens.

halogens