Self-study
Failure to rotate crops depletes the soil's nutrients gradually unless other preventive measures are taken. ██ ███ ██████ █████████ ███ ██████████ █████████ ██████████ █████ ██████ ██ █████ ██████ ██████████ ██ ███████ ██ ███ █████ ███ █████ ██████ █████ ██████ ██ ██████████ ███ █████ ██ ████ ████ ███ ███ ██████████ ███████ ██████ ████ █████ █████ ██ ██████████████ █████ ████ ███ ████ ██████████ ██ █████ ████ ██ █ ████████████ ████ ██ ███████ ████ ██████ ███████ ██ ███████████ ███ ████ ██████████ ████ ██ ███████████ ███ ███████ ███ ██████ █████ ██ ██████ ████ ██████ █████ ███████████
Summary
Failure to rotate crops depletes the soil’s nutrients gradually unless other preventative measures are taken. If the soil’s nutrients are completely depleted, then additional crops can be grown only if fertilizer is applied to the soil. If vegetables are grown in soil that has had fertilizer rather than being grown in non-fertilized soil, they are more vulnerable to pests. Consequently, these vegetables must be treated with larger amounts of pesticides. The more pesticides used on vegetables, the greater the health risks to humans from eating those vegetables. 
Notable Valid Inferences
The vegetables were grown in soil not completely depleted of nutrients.
7.
Suppose there were some vegetables ████ ████ █████ ██ ████ ██ █████ ██████████ ███ █████ ████ ████████ ██ ███ █████ ██ ███ ████████ █████ ███ ██ ███ █████████ █████ ████ ██ ██ ████ █████████ █████ ███████████
a
The soil in █████ ███ ██████████ ████ █████ ███ ████ ████ ██████████ ████████ ██ █████████ ███████ ██ ██ ███████ ███████ ██ ██████ ██████
Must be false. As shown by the second conditional in the diagram, soil that is completely depleted and growing crops requires the use of fertilizer. But the question stem indicates that fertilizer has not been used even though crops are growing. This implies that the soil has not been completely depleted of nutrients.
b
It is not ████████ ████ ███ ██████████ ████ █████ ██ ████ ██ █████ █████ ███ ████ ████████
Could be false. We have no information that implies the soil cannot have been rotated. The first statement, which is the only one that mentions crop rotation, doesn't allow us to conclude that soil has not been rotated. It tells us what happens if soil has not been rotated, but it does not tell us what information is sufficient to conclude that soil has not been rotated.
c
The vegetables were █████ ██ ████ ████ ███ ███ ████ ██████████ ████████ ██ █████████ ███ ███ ███████████ ████ ██ █████ █████ ███ ████ ████████
Must be true. As shown below, which presents the contrapositive of the second conditional in our diagram, if fertilizer has never been used, then either the soil has not been completely depleted or there are no growing crops. Since we know from the question stem that fertilizer has never been used, but vegetables are growing, this implies that the soil has not been completely depleted of nutrients. 
d
Whatever the health █████ ██ ██████ ████ ██████ ███ ███████████ █████ █████ █████ ███ ██ ████████████ ██ ███ ███ ██ ██████████ ██ █████
Could be false. We know that the vegetables were grown in soil that has never had fertilizer applied. We do not know whether pesticides were also used. Although we know that fertilized soil must use more pesticides, this does not imply that soil that is not fertilized does not use any pesticides at all.
e
The health risks ██ ██████ ████ ██████ ███ ██████████ ████ ██ ████ ████ ███ ██████ █████ ██ ██████ ████ ██████ ███ ████ █████ ██ ██████████ ███████ ████ ███████████
Could be false. We know that the more pesticides used on crops, the greater the health risks to humans. But we do not know whether pesticides were used on the vegetables. Although we know that fertilized soil must use more pesticides, this does not imply that soil that is not fertilized does not use any pesticides at all.