How do scientists determine the number of neutrons in an isotope of an atom? a. they find the number of protons. b. they divide the atomic mass by two. c. they add the number of electrons and protons. d. they subtract the atomic number from the atomic mass.

'D' would do the job ... When you subtract the protons from the mass,
what you have left is neutrons.  (The electrons can be ignored.  It takes
around 1840 electrons ! to add the mass of a single proton or neutron !)

I don't know it for a fact, but I'd be surprised if the process is really that
simple.  I mean, it starts out with knowing the atomic mass, and then
knowing the number of protons in the nucleus.  Each of those is a
whole complex problem in itself.