Skunks primarily eat fresh plants and animal

First, we can see that the initial sentence is wrong. A "comma but" conjunction is used to join two complete sentences, but the part after the conjunction is not complete as it lacks a subject and a verb.

Second, we can see that the choice to switch from "but" to "and" is unappealing. Clearly the two ideas being expressed need a contrast word, and "and" won't get the job done.

Answer choice (E) merely repeats the error that is found in (A). Accordingly, only (D) is a good choice. "Although" provides good contrast, and the underlined portion contains a subject and a verb.

Eat and augment are the 2 parallel verbs here. Only 2 options have it of which one changes the tense. Hence correct chioce is D.

I'm glad that you were able to reach the right answer. However, I question the process by which you did.

It seems that you are of the opinion that two clauses joined by "although" must be parallel and the same tense. This is not necessarily true.

For example, one might say:

Although John was born in Tazmania, he loves British comedy. (different tenses).
Although John grew up in Boston, his wife has lived in Wales all her life. (different subjects, different tenses).

Both of the above sentences are possible.