Isobars are

Since we are aware, atoms have the same amount of nucleons (nucleons are the species present inside the nucleus of the atom). In plain English, we may state that isobars are several chemical elements that share the same mass number but have distinct atomic numbers. As a result, we must now explain isobar using an example.

Let's talk about the idea of isobars as follows: Isobars are chemical elements with distinct atomic numbers but the same mass number. Atoms with the same number of nucleons (nucleons are the species present inside the nucleus of the atom) are referred to as isobars.
The name "isobar," which is derived from the Greek words "isos" (which means equal) and "baros" (which means weight), was proposed by Alfred Walter Stewart in 1918.

Due to the varied electron counts, these isobaric elements or species exhibit various chemical characteristics. The isobars have various atomic numbers but the same atomic mass due to differences in the amount of neutrons that combine with protons to produce the same total number of nucleons. The following is an illustration of an isobar:

${}_{18}A{r}^{40}{{}_{19}K}^{40}and{}_{20}C{a}^{40}$are the isobars of each other as the atomic mass (or the number of nucleons) of each of them is 40 which is written in the superscript and their atomic number is different which is shown in the subscript. As their atomic numbers are different therefore their chemical properties are also quite different from each other.

Hence, option A is correct.