Explanation

Hydrogen does have isotopes: protium (1H, no neutron), deuterium (2H, one neutron) and tritium (3H, two neutrons, radioactive). Hence the statement 'Hydrogen has no isotopes' is not correct. The other statements are true: hydrogen is an element, it is the lightest element (atomic number 1), and a hydrogen-oxygen mixture is explosive (as in the oxyhydrogen reaction). The incorrect statement is (C).