Cyrus generally sells 50 drives extra every month. Option b

Similarly, Diya sells 10 extra drives every month.

In March, either 35 extra drives are sold instead of 20 or 50 extra dries are sold instead of 35.

If, 35 extra drives were sold instead of 20, the total number of drives would be

500+520+555+(555+20)+(555+40).......

= 1020+10/2(2*555+9*20)=7470

But noone has sold 7470 drives by the end of the year. If 50 extra drives were sold instead of 35, the total number of drives would be

500+535+585+(585+35)+(585+70)...= 8460 drives

Arundati generally sells 35 extra drives and 50 drives instead of 35 are sold in March. Similarly, Bhavna generally sells 20 extra drives and sells 35 extra drives instead of 20 in October and Diya generally sells 10 extra drives and sells 20 extra drives instead of 10 in July