1 – Correct
Since Cobalt-60 decays by beta, its atomic number increases by 1 and mass number stays the same.
Radiation by gamma does not affect the mass number nor the atomic number.
So after beta decay, the atom’s mass number would be same = 59
Atomic number would be +1 = 27 + 1 = 28.
So Cobalt-60 is converted to ₂₈Nickel.
2 – Correct
Since Cobalt – 60 decays by gamma, the tumour is attacked by gamma.
3 – Wrong
The radiation attacks all the cells and not just the malignant cells.
Since statements 1 and 2 are correct, D is the answer.