Answer
(a) Cosine function
(b) Linear function
Work Step by Step
(a) The cosine function might be chosen as a model for the data since, at $x=0$, cosine functions begin at their amplitude and oscillate periodically, which seems to approximately fit the data points. It must be slightly shifted vertically to fit the points.
(b) The linear function might be chosen as a model for the data since the data points approximately form a straight line with some negative slope.
