Answer:
The correct answer is (c) the poll uses voluntary response, so the results tell us little about the population of all adults.
Step-by-step explanation:
You can't conclude (a) because this question hasn't be made to all the Americans.
You can't conclude (b) because the poll doesn't use any convenience for the selection of the sample
You can't conclude (d) because this is not a poll with a premeditated sample, therefore, the size is inconclusive.
You can conclude (c) because the poll was available for anyone who voluntary wanted to answer it.