CGPET 2019 Solved Paper

On emission of each α -particle, atomic number of an element decreased by 2 units and atomic mass by 4 units, while on emission of each β -particle, atomic number is increased by one unit and atomic mass remain unaffected.
Let initial atomic number of radioactive species (A)=Z and atomic mass of (A)=M, then,
AZM

−α

─────▸

BZ−2M−4

−α

─────▸

CZ−4M−8

−4β

─────▸

DZM−8
Also,
Isotopes are the species which have same atomic number but have different atomic masses while isobers have same atomic mass and different atomic numbers.
The species with same number of neutrons but have different number of protons are called isotones.
Thus, species A and ′B′ are isotopes while species C and D are isobars.
Hence, option (b) is the correct answer.