Answer:
The author uses the purchase of the trees to show that this family is determined to save the farm.
Step-by-step explanation:
Perseverance is when a person boldly and bravely hanging on into the thing he's doing despite the many difficulties and obstacles he may face. This act of looking and trying to succeed despite the problems that may come shows the strong will and unnerving spirit of the individual.
In the given excerpt from the text "Black Day, 1935", the family's perseverance is seen when the father revealed he had "put in [a] tree order". His statement that "this farm is not going down without a fight" explicitly reveals the family's decision to stay put in the farm and brave the blizzard.
Thus, the correct answer is the fourth option.