15) It can't be an exponential or logarithmic function because the function increases and decreases. Exponential functions always only increase or decrease. Logarithmic functions always increase until they reach a carrying capacity, where it then stabilizes.
16) You can find the minimum possible degree polynomial by finding how many times the graph touches the x-axis. In this graph, it touches the x-axis 3 times, but the graph kind of makes a parabolic shape at x=7, so you would have to count that as touching the x-axis 2 times. Therefore, the answer is a 4th degree polynomial.
17) The x-intercepts are when the graph touches the x-axis. It touches the x-axis at x=0, 7, and 12. This means that the population of the insects is 0 when at the beginning of the 12 month period, after 7 months, and after 12 months.