Atypical carbon atom has six protons, six neutrons, and six electrons. which of these has the least amount of mass?
a) one proton
b) one neutron
c) six electrons
d) these masses are too tiny to measure

One proton or one neutron have very close to the same mass.

Their mass is about the same as the mass of 1,840 electrons !
So six electrons have roughly the same mass as 0.0033 of one
proton or one neutron.

Yes, these masses are very tiny.  None of them was ever measured
until the 20th Century.  But all of them have now been measured,
very precisely and accurately.  This is very impressive, and should
give us a lot of admiration for scientists of the past hundred years.
It would be very interesting to look around online and find out
how they did it.