From the graph, it is obvious that the trend is decreasing from 100 on day 2, to 1 on day 10. So, the answer could either be A or C. The question would be how fast is it decreasing? To illustrate this, let's find the difference of consecutive data:
100 - 26 = 74
26 - 6 = 20
6 - 2=4
2-1=1
It must not be an additive rate because there is no common difference. Let's illustrate if the trend is in multiplicative rate:
100/26 = 3.85
26/6 = 4.33
6/2 = 3
2/1 = 2
More or less, they have a common divider. Hence, the decreasing rate is in multiplicative rate. The answer is A.