A function is a relation in which each element of the domain, or x-value, is mapped to one element of the range, or y-value.
The question we are asked is if cost is a function of the activity type. This means that cost would be the range and activity type would be the domain.
If this is a function, then each activity type would have only one cost. However, on different days of the week each activity type has different prices; this means there is more than one cost for each activity type, and it is not a function.