Which one of the following statements is correct?

The mass of an electron is approximately 1/1836 of the mass of a proton, which is commonly approximated as about 1/2000, making statement (b) correct. Statement (a) is incorrect because a neutron is not literally formed by combination of an electron and a proton; it is a fundamental subatomic particle (though it can decay into a proton and electron). Statement (c) is wrong because Cobalt-60 is used in cancer treatment, not goiter (goiter is treated with iodine, often radioactive iodine-131). Statement (d) is incorrect because J.J. Thomson proposed the 'plum pudding' model where electrons are embedded in a sphere of positive charge; he did not propose a nucleus made of neutrons.