We are given:

First step is to get rid of fractions. We start by multiplying both sides by a.

Now we cancel a on right side and we multiply both sides by c.

After canceling c on the right side we are left with:
I think it would be D. A decreasing exponential function.
So
a. so 100-1/10 of 100 or 100-10=90
then second dose minus 10% or 90-9=81
third dose 81 minus 10%=81-8.1=72.9
fourth dose 72.9 minus 10% or 72.9-7.29=65.61
so after 4th dose only 65.61% are left
b. so to get it to 5% you do alot and find that it takes 27 tries to get the percent down to 5.2% and 28 doses to get it to 4.7%