When you enumerate you need to put comas between all the items except the ones which are connected by "and" : as A is not correct as it does not have comas, and B is incorrect as it has comas where "and" is.
now, in a way both C and D are correct, but C is the usual accepted version, and D is an example of "Oxford coma" (where the coma is optional, to remain on the safe side, choose C).
The answer is B, The relative pronoun is that; it functions as part of the adverb clause.
A. The beauty of nature, a delicious treat.
Because he compares nature with a treat.