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A professional puppeteer could control all the puppet and tell stories on the basis of classic such as Faust. So mastering literature, as well as sculpting, painting puppets and making costumes, ______ essential in running a puppet theater.
（A）were
（B）was
（C）which were
（D）which was
The answer is（B). I want to ask that why the answer is "was", and why the others are wrong?
The rule that is being applied here is that the subject of that clause is "mastering literature", which is singular, so the verb is "was".
The phrase ", as well as sculpting, painting puppets and making costumes," is an additional phrase but not part of the subject. It would look more obvious if written as "...mastering literature (as well as sculpting, painting puppets and making costumes) was...". If you remove the part in brackets, the sentence should remain grammatically correct.
But I would tend to use "were" in this situation, treating the subject as the plural phrase. This is an example of when the subject gets separated from the verb, the verb form tends to adapt to the words that are close to it.
"Which was" and "which were" don't work; they would leave the sentence having no main clause, which is ungrammatical.
TECHNICAL SUPPLEMENT to James K's answer
This is not a "complex subject". As well as, in formal use, does not act as a coordinating conjunction†; it's more or less equivalent to in addition to.
The phrase as well as sculpting, painting puppets and making costumes is thus a supplement to the main clause, bracketed with commas to mark it as parenthetical. It could be moved to the beginning or end of the sentence (or even omitted entirely) without damage to the sense:
So as well as sculpting, painting puppets and making costumes, mastering literature was essential in running a puppet theater.
So mastering literature was essential in running a puppet theater, as well as sculpting, painting puppets and making costumes.
So mastering literature was essential in running a puppet theater.
† But you will find many writers using it this way even in formal contexts—it is evolving into an 'elegant variation' of and. I do not recommend emulating these writers.
