When the masses of an electron, proton, and neutron are $$\text m_\text e,\text m_\text p$$​, and $$\text m_\text n$$​, respectively, then the nuclear mass of an atom $$_\text Z\text X^\text A$$​ is _________.

Correct option is B

Given 

​$$\begin{aligned}m_e & = \text{Mass of electron} \\m_p & = \text{Mass of proton} \\m_n & = \text{Mass of neutron} \\Z & = \text{Atomic number (number of protons)} \\N & = \text{Number of neutrons}\end{aligned}$$

$$\text{The mass number } A \text{ of an atom is the sum of the number of protons } Z \text{ and the number of neutrons } N:$$​$$A = Z + N$$

$$\text{The nuclear mass of an atom is given by:}$$

$$\text{Nuclear Mass} = Z m_p + N m_n$$​​​​​