Which of the following statements about hydrogen are correct?

a.  Hydrogen has three isotopes of which protium is the most common

b.  Hydrogen never acts as a cation in ionic salts

c.  Hydrogen ion, H+, exists freely in solution

d.  Dihydrogen does not act as a reducing agent

Choose the correct option
1. (a, b)
2. (b, c)
3. (c, d)
4. (a, d)

Among the three isotopes of hydrogen, protonium is the most common. In ionic salts, hydrogen exists as hydride (H-).