Explanation

(a)
Ortho and para hydrogens are two forms of hydrogen which differ only in direction of spin of proton.
Protium $\left( \ _{1}^{1}H \right),$ deuterium $\left( \ _{1}^{2}D \right)$ and tritium $\left( \ _{1}^{3}T \right)$ are three isotopes of hydrogen. All of them have one proton and electron each. Protium has no neutron, deuterium has one neutron and tritium has two neutrons.