To get $\mathrm{n}$-type doped semiconductor, impurity to be added to silicon should have the following number of valence electrons :

Doping of group 14 elements (such as $Si$, $\mathrm{Ge}$, etc.) with elements of group 15 (such as As) produces an excess of electrons in the crystals, thus, giving n-types semiconductors.

So, to get n-type doped semiconductor, impurity to be added to silicon should have 5 valence electrons.

Hence the correct answer is (B) 5