The correct answer to this question is A. The nucleus of the atom was discovered by Ernest Rutherford in the year of 1911. Born in New Zealand, Rutherford is a British physicist considered as the father of nuclear physics. He discovered the atomic nucleus based on the Geiger–Marsden gold foil experiment from 1909, where Rutherford directed Hans Geiger and Ernest Marsden which used a thin gold foil to discover the nuclear nature of atoms.
Rutherford's interpertation of the data allowed him to create Rutherford modle of the atom, concluding that every atom possesses a nucleus holding its positive charge and majority of its mass, orbited around low-mass electrons.