The correct answer is - A. Death rate.
We get the fertility rate from the birth rate and the death rate. More specifically, from the number of the children that are born in a given time frame (usually a year) we minus the number of the people that have died in the same time frame, so the difference between them gives us the fertility rate.
The fertility rate can be positive (in the difference is a positive number) and negative (if the difference is negative number).