Answer
d. One more neutron.
Work Step by Step
The atomic number of phosphorous is 15, meaning there are 15 protons. In 31P, there are 15 protons and 16 neutrons. In 32P, there are 15 protons and 17 neutrons. Therefore, 32P has 1 more neutron.
Isotopes are variants of an element with a differing number of neutrons.