Final answer:
The equation d = 75t signifies that distance is directly proportional to time (a), and if rearranged, time is also directly proportional to distance (c). Statements b) and d) are incorrect.
Step-by-step explanation:
The equation d = 75t shows that the distance (d) is equal to 75 times the time (t), indicating a direct proportional relationship between distance and time. As time increases, distance increases at a constant rate, and vice versa. Therefore, the correct statement is a) Distance is directly proportional to time. Additionally, since the distance is dependent on the time, statement c) Time is directly proportional to distance also holds true if we rearrange the formula to t = d/75, symbolizing that time increases with the increase in distance. However, this arrangement depicts time as the dependent variable, which is a reflection of the direct proportionality and not a primary characteristic of the relationship expressed by the equation d = 75t. Statements b) and d) are incorrect because they suggest an inverse relationship, which is not the case here.