Isotopes are the atoms of the same elements having same atomic no(i.e. the sum of the electrons and protons) but different mass no(i.e. sum of the protons and neutrons).

Isotopes, Isobars, Isotones, and Isochores

- Isotopes: Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This means that isotopes have the same atomic number but different mass numbers. Isotopes can have different physical properties and can exhibit different levels of stability.

- Isobars: Isobars are atoms of different elements that have the same mass number but different atomic numbers. Since the atomic number determines the identity of an element, isobars are different elements with different chemical properties. However, they have the same mass number, which means they have the same total number of protons and neutrons.

- Isotones: Isotones are atoms of different elements that have the same number of neutrons but different numbers of protons. This means that isotones have the same neutron number but different atomic numbers. Isotones may have different chemical properties due to their different atomic numbers, but they have the same number of neutrons.

- Isochores: Isochores are regions of DNA or chromosomes that have a similar base composition. They are not related to the concepts of isotopes, isobars, or isotones in atomic structure.

Explanation of the given options:

- 21H and 31H: These are both hydrogen atoms, with the number indicating the mass number (the sum of protons and neutrons). Since they have different mass numbers, they are isotopes of hydrogen.

- Option 'A': Isotopes. Based on the explanation above, 21H and 31H are indeed isotopes because they are atoms of the same element (hydrogen) with different mass numbers.

- Option 'B': Isobars. Isobars would be atoms of different elements with the same mass number. Since both 21H and 31H are hydrogen atoms, they cannot be isobars.

- Option 'C': Isotones. Isotones would be atoms of different elements with the same number of neutrons. While 21H and 31H have different atomic numbers and different numbers of neutrons, they cannot be considered isotones.

- Option 'D': Isochores. Isochores are not related to atomic structure or the concepts of isotopes, isobars, and isotones. Therefore, this option is not applicable to the given atoms.