The types of things most people had to go without during the Depression (C) is where Matthew's great-grandmother is most likely to help.
The reason this is the only right answer is that we are told that she was a child in the early 1930s, at the time of the Great Depression. A child's perception of her living conditions is based on memories of what she felt or did. But she was too young to know:
- how many Americans were unemployed at the time (A), and presumably did not understand very well what a "percentage" meant;
- similarly, a child is too young to grasp what "stock market points" represent (B);
- and finally, she probably was not aware of the laws which led to the crisis (D).
However, it is possible for a child to remember the things she was missing in her life because of the difficult economic situation. That is why Matthew's great-grandmother will most likely be able to answer question C.