Final answer:
The equation that represents the total number of students on a field trip being proportional to the number of buses is t = sb, where t is the total number of students, s is the number of students per bus, and b is the number of buses.
Step-by-step explanation:
The total number of students on a field trip (t) is proportional to the number of buses (b), with each bus carrying the same number of students (s). To represent this situation with an equation, we can say that the total number of students (t) is equal to the product of the number of students per bus (s) and the number of buses (b). Therefore, the equation that best represents this situation is t = sb.