B.) The word "But" should not be capitalized.
Note that the rule with capitalization is that proper nouns and words beginning sentences should be capitalized. When we look at the original sentence, it reads as follows:
The tracking of online activity is not just a privacy issue, but an economic and political minefield.
We can see that the word “but” is not the beginning of a sentence. In addition, “but” is not a proper noun. In this particular sentence, “but” is part of the correlative conjunction pair “not/but.” Just because the word “but” is beginning the second part of the quotation and appears immediately after quotation marks, it’s important not to get confused and think that it is beginning a new sentence. Therefore, it should not be capitalized.