The brakes of a speeding truck are slammed on and it skids to a stop. If the truck

mass number is defined as sum of number of protons and neutrons

so here for Isotope A

Isotope A contains 56 protons and 80 neutrons.

Mass number will be given as

For Isotope B

Isotope B contains 55 protons and 81 neutrons.

For Isotope C

Isotope C contains 57 protons and 80 neutrons.

For Isotope D

Isotope D contains 56 protons and 74 neutrons.

So here Isotope A and Isotope B has same mass number