This question tries to confuse us with the word "each" . Each when used before the subject ("each of the three largest carriers" is singular subject) makes the subject singular but when each follows the subject as in this case it does not change the subject to singular .The subject remains plural
Winship estimates that the three largest carriers - Delta, United, and American - each generate roughly $1 billion annually from the sale of frequent-flier miles to hotels, rental-car companies, and others offering points-for-miles to their customers
A) generate roughly $1 billion annually from the sale of frequent-flier miles to -
Correct answerB) generates roughly $1 billion annually from the sale of frequent-flier miles to -
Singular verb (generates) is wrong in this case because the subject is carriers(plural)C) from the sale of frequent-flier miles generates roughly $1 billion annually per year, -
Same error as Option BD) from the sale of frequent-flier miles generate roughly $1 billion annually for -
Changes meaning . These carriers dont generate money for hotels E) generate roughly $1 billion annually from the sale of frequent-flier miles,
Changes meaning carriers dont generate money by selling hotels rental car companies etc.