A. there is a tendency for people to view Trotsky's violent acts as

In the version created with this choice there is no referent for he. The possessive form, Trotsky's, is not a suitable referent. A non possessive form is required.

B. the violence of Trotsky tends to be viewed as

In this version also there is also no clear referent for he, which almost seems to refer to the violence of Trotsky. of Trotsky is another possessive form of Trotsky.

C. Trotsky, who also participated in violent acts, tends to be viewed by people as

In this version, he properly refers to Trotsky, and the sentence effectively conveys the correct meaning.

D. Trotsky tends to be viewed by people with violent acts as

In this version, the violent acts are incorrectly attributed to the people who view Trotsky, rather than to Trotsky.

E. there is a tendency for people to view Trotsky's violent acts to be

There is no proper referent for he and view x to be y is not idiomatically correct. The correct form is view x as y.

The correct answer is C.

Nevertheless, in the best answer, option C, there is no need for the phrase by people. Who would be viewing Trotsky's acts in a certain way, if not people? The meaning of the sentence is identical without the phrase. This kind of departure from the standards of The Official Guide is too common in Kaplan questions. By those standards, we could say that in fact this question has no correct answer.