what does it mean to be a function how can you determine functionality of a relation or eqaution graph

Determining whether a relation is a function on a graph is relatively easy by using the vertical line test. If a vertical line crosses the relation on the graph only once in all locations, the relation is a function. However, if a vertical line crosses the relation more than once, the relation is not a function.

The answer is 8 apex

The right answer is option d.