Language past parfect tense

Maybe the following explanation will answer your question.

A) This is easy to eliminate because the singular form has does not agree with the subject English, Spanish and Italian words.

B) Once again, is does not agree with words.

Two down, three to go, at least we can hope that the question is well enough constructed that that will be the case.

C) This makes sense. The verb have agrees with words. Also, it makes sense that words have been added over time, and potentially are still being added even today.

So C looks as if it's going to be the answer.

D) This is saying pretty much the same thing as C, but D is more awkward and does not make as much sense somehow.

E) In this one are does agree with words. So the type of subject verb agreement issue we saw in other choices is not present in this choice. However, this choice does not make as much sense as choice C. What the sentence seems meant to convey is not that the words are added, as in now, they are added. That does not really make sense. It's that they have been added.

So I am going with C.