The mass of an electron is

:

the same as the mass of the atom

equal to the proton mass plus the neutron mass

the smallest of the three major particles

the same as the mass of the proton

Remark

The electron has a mass that is roughly 1/1800 the size of either the proton or the electron. In comparison the electron's mass is very small comparred to the other 2.

Solution

A is totally wrong. A part of an atom can't weight the same as the atom.

B is totally wrong as well. See the remark above.

D is wrong. See the remark above

C is the answer. It's mass is quite small compared to the other 2.