Pick the correct passive form of the following sentence :
"One sees this sort of advertisement everywhere."

Option B is grammatically incorrect; the advertisement does not "see" and can easily be eliminated.

Option C is also slightly grammatically incorrect, it should be "is seen" and not "are seen", also the placement of "one" fits awkwardly. 

Option D changes the intent of the original sentence, it is not that thee advertisements are shown everywhere but that they are seen everwhere 

Option A is the only correct passive form of the sentence. 

Therefore, Option A is the correct answer.