which is the icon for an exponential trend line?

Brief Explanations:

An exponential trend - line has a curve that shows rapid growth (or decay). The first option has a wavy curve which is not typical of an exponential function. The second option has a curve that starts slow and then increases rapidly which is characteristic of exponential growth. The third option is a straight - line which represents a linear relationship. The fourth option has a curve but it may not show the typical rapid growth pattern of an exponential function as clearly as the second one.

Answer:

The second icon (from the top - left, going row - by - row)