In the original sentence, there should be a conjunction between two clauses instead of a comma.
(A) buyers can choose among bronze, silver, gold, and platinum plans, there are:
INCORRECT, instead of comma there should be a conjunction before
there are(B) because buyers can choose bronze, silver, gold, or platinum, there are:
INCORRECT, nonsensical as buyers need to choose plans not metals such as bronze, silver etc.
(C) however, buyers can choose among bronze, silver, gold, and platinum, and:
INCORRECT, nonsensical as buyers need to choose plans not metals such as bronze, silver etc.
(D) buyers can choose among bronze, silver, gold, platinum plans, but:
INCORRECT, missing AND between gold, platinum plans.
(E) although buyers can choose among bronze, silver, gold, and platinum plans, there are:
CORRECT, subordinating conjunction modifies
there are in a correct manner.
IMO E.
Bunuel
The national insurance firm offers four tiers of coverage plans;
buyers can choose among bronze, silver, gold, and platinum plans, there are no plans that offer greater coverage than the platinum tier or less than the bronze.
(A) buyers can choose among bronze, silver, gold, and platinum plans, there are
(B) because buyers can choose bronze, silver, gold, or platinum, there are
(C) however, buyers can choose among bronze, silver, gold, and platinum, and
(D) buyers can choose among bronze, silver, gold, platinum plans, but
(E) although buyers can choose among bronze, silver, gold, and platinum plans, there are