OFFICIAL KAPLAN EXPLANATIONHere the underlined section is quite long, but this question is easier than it may at first appear. The modifying phrases set off by commas are a hint to make sure the modification is logical. The introductory phrase, "More than any other single event in the history of Europe," should modify what comes after it; "the Spanish Inquisition" comes after the comma, so there's no modification error. The phrase after "the Spanish Inquisition" ("instituted ... rulers") also correctly describes the thing just before it, which is still the Inquisition. Next, in a complicated sentence structure like this, identify the subject and verb and make sure they agree. The subject is the singular "Spanish Inquisition" and the verb is the singular "is"—no error. Since there don't appear to be any errors, (A) is likely the answer, but check the others to be sure.
(B) changes the present tense verb "is" to "was," which incorrectly changes the meaning of the sentence to imply that the Inquisition is no longer remembered.
In (C), one modifying phrase ("more ... Europe") is separated from the noun it modifies by another lengthy phrase ("instituted ... rulers"), creating a modification error.
(D) and (E) change the singular verb "is" to "are," creating subject-verb disagreement.
(A) is indeed the correct answer.
TAKEAWAY: Don't fix errors that aren't there. Sometimes (A) is correct!