The answer should be A.
basically, what i did was first find out what number the scientific notation equaled. because the exponent over 10 is negative, that means the decimal in the 9.9 will move 2 spaces to the right leaving us with a total of 0.099. Then, i saw what whether the rate would be in feet or inches.
first using feet i realized that because we have to see whether the rate is daily or monthly, we would have to divide this number (3) by 365 or 12. 3 divided by 365 is 0.008 which is nowhere near 0.099 (the number we want). 3 divided by 12 is 0.25 which is also not the number we want. so basically the answers cant be c or d because we wont be using feet in our rate.
as for inches, there are 36 inches in 3 feet (the total erosion rate per year). now we must see whether it is daily or monthly. 36 divided by 12 (months) is 3 which is not the number we want so basically 36 divided by 365 is 0.099.
hope that made sense