IAI May 2014 Q. 11

I have a query in this question:

The fall in a firm’s SRAC with an increase in production would be due to which of the following?

a. The greater divisibility of fixed costs
b. Diminishing returns to a fixed factor
c. Economies of scale
d. Diseconomies of scale

Why is the answer a and not b?

Thanks
Regards,
Shyam


Edit: Got the answer!