Concentration of germs per harmful substances in 150 liters of
$$\frac{0.3}{100}\cdot100\%=0.2$$
Concentration of germs must be less than 0.09% for water to be considered safe
1 gm will reduce concentration of germs by half.
1 gm will reduce 0.2
$$\frac{1}{2}\cdot0.2=0.1$$

0.1% is still greater than 0.09% hence we will add another 1 gm to reduce to 0.1%
1 gm will reduce to 0.1
$$\frac{1}{2}\cdot0.1=0.005\%$$
0.05% is lesser than 0.09%, water is now safe

2 gm of chlorine was needed to purify the water.
$$answer\ is\ Option\ B$$