27) D

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.