The mass of 1 mole of electron is:
A. $0.55mg$
B. $9.1 \times \mathop {10}\nolimits^{ - 31} g$
C. $9.1 \times \mathop {10}\nolimits^{ - 27} g$
D. $1.008g$

Hint: We know that electrons are the smallest subatomic particle which revolves around the nucleus and is the lightest subatomic particle known.

Complete step by step solution:
We already know that electron is the lightest subatomic particle and it carries negative charge of $1.6 \times \mathop {10}\nolimits^{ - 19} C$ which is also considered as the basic unit of electric charge termed as electronic charge and was discovered by J.J Thomson.
The rest mass i.e. mass of a stationary electron is approximately equal to $9.1 \times \mathop {10}\nolimits^{ - 31} kg$.
The other two subatomic particles of atoms are proton and neutron.
We know that proton is positively charged subatomic particle having mass equal to $1.673 \times \mathop {10}\nolimits^{ - 27} kg$ whereas neutron is electrically neutral and has mass equal to $1.675 \times \mathop {10}\nolimits^{ - 27} kg$.
Now by comparing the mass of an electron with that of a proton, we can say that it is only $\dfrac{1}{{1836}}$ the mass of the proton. Therefore, an electron is considered nearly massless in comparison to proton and neutron and that’s why electron mass is not counted while calculating the mass number of an atom.
It is known to us that mole is the SI unit of measurement of amount of substance.
We already know that, for any particle $1mol = 6.022 \times \mathop {10}\nolimits^{23} $
Also, Mass of 1 electron $ = 9.1 \times \mathop {10}\nolimits^{ - 31} kg$.
Therefore, mass of 1 mole of electron $ = (6.022 \times \mathop {10}\nolimits^{23} ) \times (9.1 \times \mathop {10}\nolimits^{ - 31} )
                                                                     = 54.80 \times \mathop {10}\nolimits^{ - 8}
                                                                     = 5.48 \times \mathop {10}\nolimits^{ - 7} kg
                                                                      = 5.48 \times \mathop {10}\nolimits^{ - 4} g \approx 0.55mg$
Therefore, on the basis of the above calculation, option A is the correct answer.

Note: We should know that electrons are very important for bonding of individual atoms together.
If electrons were not present then atoms would not be able to interact in many reactions and would not be able to form molecules of different types.