Select the pairs of elements that would be expected to have similar properties and reactivity?

Elements in the same group of the Periodic Table would have similar chemical properties.
a) Both Li and K belong to Group IA.
b) Ar belongs to Group 0, while Br belongs to Group VIIA.
c) Both F and I belong to Group VIIA.
d) S belongs to Group VIA, while Cl belongs to Group VIIA.
e) Both C and Si belong to Group IVA.
f) Rb belongs to Group I, while Sb belongs to Group VA.

Hence, the answer is : a), c) and e)

It would be a) Lithium and Potassium.

The reason being is that they are both elements in the group IA series known as the alkali metals. They are highly reactive metals that can form a strong base with the addition of a hydroxide for instance.

Argon and Bromine are dissimilar because Argon is a noble gas that has a complete octet so it isn't electronegative and also minimally reactive. Bromine is the opposite because it is a highly reactive non metal with a high electronegativity that can form a strong acid with the addition of a Hydrogen ion.

Although Flourine and Iodine are in the same group, Flourine is more electronegative than iodine. It is the MOST electronegative element on the periodic table since it is that close to filling its outer shell to get a full octet. However, Flourine cannot form a strong acid with a Hydrogen ion whereas Br, Cl, and I can. It has to do with the fact that increased eelectronegativity in a covalent bond actually weakens the bond; hence HF being a weak acid.

Sulfur and Chlorine do not pair because sulfur is a least reactive non-metal and chlorine is a highly reactive non-metal (due to electronegativity).

Silicon is a metalloid which has properties of a metal and non-metal. Carbon is a minimally reactive non-metal which is also one of the fundamental elements found in the human body.

Rubidium and antimony have completely opposite properties....

All you have to do is take a look at the periodic table.