Number of electrons present in the species H2+, He, H2 and O2+ respectively are:

H2+ is a hydrogen molecular ion with 2 protons (2 H atoms) but with one electron removed, so it has 1 electron. He (helium atom) has 2 electrons. H2 (hydrogen molecule) has 2 hydrogen atoms each contributing 1 electron, giving a total of 2 electrons. O2+ is the oxygen molecular cation with 2 oxygen atoms (8 electrons each = 16 total) minus one electron removed to form the cation, giving 15 electrons. Therefore, the number of electrons are 1, 2, 2, and 15 respectively, matching option (a).