Hich of the following is the most credible, unbiased secondary source for information about immigration rates in the u. s. today?
a. a book written by a harvard university sociologist
b. a book written by a political candidate
c. a book written by a corporate agricultural company
d. a book written by the son of an immigrant

The most credible and unbiased secondary source for information about immigration rates in the US would be A - A book writen by a Harvard University sociologist. 

The reason why this is the correct answer and not C is because C is in no way credible or has enough information (it is not the speciality of an agricultural company to talk about immigration rates). Neither is a book written by a son of an immigrant unbiased as something like this can heavily skew one's perception, and the same goes for a book written by a political candidate - this can also skew the information which a book contains. 

For that reason A  is the correct answer.