$7 g$ of nitrogen occupies a volume of 5 litres under certain conditions. Under the same conditions one mole of a gas, having molecular weight 56 , occupies a volume of

number of moles= $\frac{Givenweight}{molecularweight}$

Number of moles of nitrogen= $\frac{7}{28}=0.25$

Since 0.25 moles of nitrogen occupies 5 litres.

Therefore 1mole if nitrogen occupies = $\frac{5}{0.25}=20$

Hence the correct option is (B).