Which of the following is an example of a pair of Isobars?

Calcium and argon

The correct answer is option 2. Calcium and argon.

Isobars are atoms of different chemical elements that have the same mass number, which is the total number of protons and neutrons in the nucleus of the atom. However, isobars have different atomic numbers, representing the number of protons specifically.

In the case of calcium and argon:
- Calcium (\(^{40}_{20}Ca\)) has an atomic number of 20 and a mass number of 40.
- Argon (\(^{40}_{18}Ar\)) has an atomic number of 18 and a mass number of 40.

Both calcium and argon have a mass number of 40, indicating that they have the same number of protons plus neutrons in their nuclei. However, they have different atomic numbers, meaning they have different numbers of protons.

This distinction in atomic number means that calcium and argon are different chemical elements with distinct chemical and physical properties. Despite this difference, they are isobars because they share the same mass number.