The answer to your question would be that the underlined group of words is an essential clause. That is, the correct option would be D.
A clause consists of a subject and a verb and is the smallest grammatical unit that expresses a thought. In this case, the clause "that can come from litter" is an essential relative clause because the information it provides is considered relevant to the meaning of the sentence. This information is considered necessary to identify the noun modified by the clause. If it was dropped, the meaning of the sentence would be altered (a)
(a) *Sea glass is perhaps the only beautiful thing