Question

Arabian glass, which authorities believe appeared in the second half of the thirteenth century, may have been inserted as decoration in marble, stone, or plaster.

Classify the underlined clause as essential or nonessential, then identify how it functions in the sentence (i.e., as what part of speech it is).
A.
nonessential, adjective
B.
nonessential, noun
C.
essential, adverb
D.
essential, adjective