All professors in the United States
All history professors in the United States
All history professors at all California State Universities
All professors at all California State Universities
Explanation
Correct answer: C.
Choice C is correct. Selecting a sample at random when conducting a survey allows the results to be generalized to the population from which the sample was selected, but not beyond this population. In this situation, the population that the sample was selected from is history professors from the California State Universities. Therefore, the largest population to which the results of the survey can be generalized is all history professors at all California State Universities.
Why the other choices are wrong
Choice A
Choice A is incorrect. Since the sample was selected at random from history professors from the California State Universities, the results of the survey can't be generalized to all professors in the United States. This population may use different texts and therefore may name different publishers.
Choice B
Choice B is incorrect. Since the sample was selected at random from history professors from the California State Universities, the results of the survey can't be generalized to all history professors in the United States. This population may use different texts and therefore may name different publishers.
Choice D
Choice D is incorrect. Since the sample was selected at random from history professors from the California State Universities, the results of the survey can't be generalized to all professors at all California State Universities. This population may use different texts and therefore may name different publishers.