The meeting was productive however, several key decisions were postponed until next week.
Correct: B
Option B is the correct choice. 'The meeting was productive' and 'several key decisions were postponed until next week' are both independent clauses. A semicolon is needed to join two independent clauses when they are related in thought. When an adverbial conjunct like 'however' connects two independent clauses, it is preceded by a semicolon and followed by a comma. Option A creates a comma splice, incorrectly joining two independent clauses with only a comma. Option C uses a colon incorrectly; a colon typically introduces a list, an explanation, or a quotation. Option D incorrectly runs the two independent clauses together without proper punctuation.