Question focus: Transitive verbs.
TOEFL loves this kind of question!!
This one is tricky because have called requires an object and a complement. Usually TOEFL likes to use call with two nouns after it. This is what I tell my students: call + two nouns.
So, in this sentence, we have a simple sentence like this:
... critics... have called Mark Twain... writer
We have two nouns: Mark Twain and writer for the verb have called.