Answer :
'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.