Answer: c. 5 bolts.
Step-by-step explanation:
From the graph we see that the opportunity cost of making 80 nails is 20 bolts.
Opportunity cost of making 1 nail is;
80/80 : 20/80
1 : ¼
If 1 nail has an opportunity cost of ¼ bolts then making 20 extra nails would cost;
= 20 * ¼
= 5 bolts.