To determine which type of graph this is, look at the estimated population, and look at the differences between each point:
89, 55, 34, 21, 13
Differences: 34, 21, 13, 8
The differences are all different values, so linear can be removed.
Because the graph is decreasing at a rate, we can eliminate both quadratic answers, as they would have increasing values.
The answer is 'exponential; y = 89 * 0.62^x'.