Answer:
D. The cost of purchasing avocados for $1.75 per pound plus a shipping fee of $0.16 per pound.
Step-by-step explanation:
A linear relationship is of the form
, where,
is the unit rate and
is the fixed value (constant).
For a proportional relationship, the value of
and thus it is of the form 
Let us check each option and express it in the form above.
Option A:
Given:
Unit rate of purchasing a basket of apples, 
Fixed price for the basket, 
Since,
, therefore, it is not a proportional relationship.
Option B:
Given:
Unit rate of purchasing a banana, 
Fixed price for the box, 
Since,
, therefore, it is not a proportional relationship.
Option C:
Given:
Unit rate of purchasing a lime, 
Discount from total cost, 
Since,
, therefore, it is not a proportional relationship.
Option D:
Given:
Unit rate of purchasing an avocado = 
Unit rate of shipping = 
Therefore, total cost per pound is the sum of the unit rates of purchasing and shipping. So,
Total cost of avocados per pound, 
There is no fixed cost on this. So, 
Since,
, therefore, it is a proportional relationship.
Therefore, the correct option is D.