Final answer:
The radioactive isotope 32P has one more neutron than 31P, therefore option (b) is correct.
Step-by-step explanation:
Compared with 31P, the radioactive isotope 32P has one more neutron. Both isotopes have the same number of protons, which is 15, since they are phosphorus isotopes (Z=15). Isotopes of an element differ in the number of neutrons within the nucleus. The mass number of 32P is greater by one unit than that of 31P, indicating the presence of one additional neutron. Therefore, option (b) one more neutron, is correct.