Question

Which of the following pairs are isotopes
(A) Ice and steam
(B) Hydrogen and deuterium
(C) Nitric oxide and nitrogen dioxide
(D) Oxygen and ozone

Answer 1

Isotopes are two or more types of atoms that have the same atomic number and position in the periodic table, and that differ in nucleon numbers due to different numbers of neutrons in their nuclei. While all isotopes of a given element have almost the same chemical properties, they have different atomic masses and physical properties.

Complete answer:
Isotopes are variants of elements that differ in their nucleon numbers due to a difference in the total number of neutrons in their respective nuclei.
There are three isotopes of the element hydrogen: hydrogen, deuterium, and tritium. Hydrogen has no neutron, deuterium has one, and tritium has two neutrons. The isotopes of hydrogen have, respectively, mass numbers of one, two, and three. Their nuclear symbols are therefore $^1H{,^2}H{,^3}H$ .
Deuterium is one of the isotopes of hydrogen. Because deuterium contains a neutron, it is more massive or heavier than protium, so it is sometimes called heavy hydrogen. There's a third hydrogen isotope, tritium, which may also be called heavy hydrogen because each atom contains one proton and two neutrons.
So, the correct answer is (B) hydrogen and deuterium.

Note:
The number of protons within the atom's nucleus is called atomic number and is equal to the number of electrons in the neutral atom. Each atomic number identifies a specific element, but not the isotope; an atom of a given element may have a wide range in its number of neutrons. The number of nucleons in the nucleus is the atom's mass number, and each isotope of a given element has a different mass number.