Answer:
B. 200 doses
Step-by-step explanation:
Given,
1 dose is required for 100 mg,
Since, 1 mg = 0.001 g,
⇒ 100 mg = 0.1 g
⇒ 1 dost is required for 0.1 g,
Thus, the ratio of doses and quantity ( in gram ) is 
Let x be the doses required for 20 grams,
So, the ratio of doses and quantity is 


Hence, 200 doses can be obtained from 20 grams of the drug.
Option 'B' is correct.