Asecure food supply depends on all of the following except?
choose one answer.
a. maintaing healthy soil and water
b. increased use of fertilizer
c. safe distribution of food
d. biodiversity

Let's  look at the options

a. maintaining healthy soil and water

This seems like an obvious option, since healthy soil is necessary to produce food. If a soil is not healthy, it will not have enough nutrients to produce plants. 
b.   increased use of fertilizer

Actually, the use of fertilizer is not necessary, as healthy soil will be enough to produce food. And increasing the use of fertilizer is even more not good, because it can be unhealthy and it can lead to the soil being infertile.

c. safe distribution of food

This is also very important. Imagine if you can produce healthy food, but you cannot deliver it safely to people and the food gets infected with bacteria on the way - then the whole effort is in vain!
d. biodiversity

This is actually very important, because if you have only one species, if this species is hit by a plague, you could loose all your crop! So you need to  keep a variety of crops