Finding what elements have the most similar chemical properties is easy. All you need to do is look at the Periodic Table. Elements in the same groups (columns) have very similar chemical properties. This is due to that fact that elements in the same group have the same number of valence electrons (electrons in the outermost shell).
Let's try an example.
Which two elements have the most similar chemical properties?
(1) beryllium and magnesium
(2) hydrogen and helium
(3) phosphorus and sulfur (4) potassium and strontium
Answer: The correct answer 1: beryllium and magnesium
Both elements appear on the same group (II),and therefore have the most similar chemical properties.