What is the total number of electron pairs shared between the two atoms in an O2 molecule?

(1) 1 (3) 6

(2) 2 (4) 4

The answer is (2) 2 pairs. The oxygen atoms combine to get stable structure. So after the combination, each atom needs to have 8 electrons. So when they share 4 electrons can satisfy this requirement. Then there are 2 pairs shared.