Final answer:
The expression for n(d), representing the number of people who hear the rumor on the dth day, is 4^d. Each day, the number of people who hear the news is multiplied by 4.
Step-by-step explanation:
The function n(d) represents the number of people who hear the rumor on the dth day. In this case, each person who hears the news tells 4 other people. So on the first day, the number of people who hear the news is 4 (assuming Susanna's 4 friends are the only ones who heard the news). On the second day, each of the 4 friends tells 4 new people, resulting in a total of 4 * 4 = 16 people hearing the news. On the third day, each of the 16 people tells 4 new people, resulting in a total of 16 * 4 = 64 people hearing the news. This pattern continues, with the number of people hearing the news being multiplied by 4 each day. Therefore, the expression for n(d) is 4^d.