Mass numbers of Li, Be and B are 7, 9 and 10 respectively. Which of the following has two electrons, three protons and four neutrons?

Li has 3 electrons but Li⁺ has only 2 electrons. Mass number of Li is 7 and the proton in Li is 3, therefore number of neutron is:

$$\begin{gathered} \mathrm{Neurton}=\mathrm{Mass} \mathrm{number}-\mathrm{proton} \\ =7-3=4 \end{gathered}$$