What is the 6th term in the sequence described by the following recursive formula?
a1=9
an=an−1−4

D) The relation R is a function because for each input there is exactly one output

There's not much else to say other than what choice D already does. Visually we can use the vertical line test to confirm if we have a function or not. If we can pass a single vertical line through more than one point on a curve, then the curve isn't a function. The graph is said to have failed the vertical line test. Otherwise, the relation passes the vertical line test and we have a function.