Answer: d) think at the margin.
Explanation:
A rational person is defined as some who makes decision with the intelligent thinking rather being emotional .This result usually results in sensible outcomes.
According to the Ten principles of Economics,it is supposed that a rational person tends to think intelligently and would prefer to make minute changes in the resources while taking decision so that optimal rate can be maintained and additional action can be further thought.
Other option are incorrect because purchasing items from smaller organization, sunk rate and average cost product equalizing with the price is not the decision ability of a rational person
