The correct option is (c).
Explanation
Isobars are atoms of different chemical elements that possess the same mass number ($A$) but different atomic numbers ($Z$). This means they have a different number of protons and electrons but the same total number of nucleons (protons plus neutrons).
Statement-wise Analysis
- Statement 1 is Correct: Isobars are characterized by having different atomic numbers ($Z$). Since the atomic number defines the identity of an element, isobars must necessarily be atoms of different elements.
- Statement 2 is Incorrect: The defining characteristic of isobars is that they possess the same mass number. If the mass numbers were different, they would not be classified as isobars.
- Statement 3 is Correct: Argon ($_{18}^{40}\text{Ar}$) and Calcium ($_{20}^{40}\text{Ca}$) are standard examples of isobars. Both have a mass number of 40, yet their atomic numbers are 18 and 20, respectively.
Key Takeaway: Isobars have the same mass number but different atomic numbers, whereas isotopes have the same atomic number but different mass numbers.