Isobars have:

Same mass number

The correct answer is option 4. Same mass number.

**Same mass number**

Isobars are atoms of different elements that have the same mass number but different atomic numbers. The mass number is the sum of protons and neutrons in an atomic nucleus. Isobars, therefore, have the same total number of nucleons (protons + neutrons) but differ in the number of protons, leading to distinct atomic numbers and chemical properties.