Which of these pairs of elements have the same number of valence electrons?
oxygen (o) and bromine (br)
selenium (se) and bromine (br)
carbon (c) and lead (pb)
oxygen (o) and carbon (c)
sodium (na) and magnesium (mg)

The same number of valence electrons exists in the same family of elements in the periodic table. In finding the answer to this problem, we just have to find the pair that belongs to the same family in the periodic table. The same family means same column. Among these, Oxygen and lead (Pb) are in the same family. This pair is the answer.

Outermost (valence) electrons