Received a PM asking me to discuss answer D. This is an OG question, so I'll have to use another sentence to discuss the general principles.
"their language is basically 18th century Chinese to which have been added Japanese, Korean, and Vietnamese words."
or
""their language is basically 18th century Chinese with Japanese, Korean, and Vietnamese words having been added to it."
"it" = ... language? Chinese? When you have a noun followed by a prepositional phrase, the prepositional phrase generally modifies that noun.
If you want the "with" to be an adverbial modifier modifying the entire previous clause, you need a comma before "with."
"Having been added" is present perfect continuous, except in modifier form. This implies an unbroken, continuous action and is generally more complex than saying "have been added." More complexity is not necessarily wrong - you just have to have support to justify using it. The general rule is to use a simpler tense unless you have reason to justify using the more complex tense. We don't really have that justification here.
Also, I'll just mention: "to which" is a very "stuffy-sounding" construction that most people do not use in daily speech - and that's probably why it's part of the correct answer - simply because a lot of people will think, "I'd never SAY it that way" and so they won't pick it.
