number of neutrons
Step 1: Isobars are atoms (not necessarily of the same element) that have the same mass number but different atomic numbers.
Step 2: The mass number is defined as the total number of protons and neutrons in an atom's nucleus.
Step 3: Isobars have the same total number of nucleons (protons plus neutrons), which results in the same mass number.
Step 4: Therefore, the defining characteristic of isobars is that they share the same mass number, aligning with option (B).