Final answer:
The standard deviation cannot be calculated without actual data on the number of people who pay very close attention to the sources cited in a news story. So, the correct answer is a. Not calculable without data.
Step-by-step explanation:
To calculate the standard deviation of the numbers of people among samples of 500 U.S. adults we would expect to say they pay very close attention to the sources cited in a news story, we need to have the data on the number of people who say they pay very close attention to the sources cited in a news story in each of the samples. Without this data, the standard deviation cannot be calculated.
So, the correct answer is a. Not calculable without data.