Answer:
D. Exponential growth.
Explanation:
We have been given that the number of people with one type of flu doubles every day for several days.
Since the number of people with flu doubles every day. This means the difference of number of people between two consecutive days will not be same, so an exponential function will represent this pattern.
Since number of people doubles every day, therefore, our function will be an exponential growth function and option D is the correct choice.