Explanation
Correct answer: D.
Choice D is the best answer. It appropriately uses a period to mark the end of one independent clause ("In 2000…settings") and the start of another ("She founded…professionals").
Why the other choices are wrong
Choice A
Choice A is incorrect. This choice results in a run-on sentence error. Both the clause before the blank ("In 2000…settings") and the clause after the blank ("she…professionals") are independent clauses, so they need to be separated by punctuation.
Choice B
Choice B is incorrect. This choice results in a comma splice error. It incorrectly joins two independent clauses with just a comma. Linking two independent clauses with a comma also requires the use of a coordinating conjunction (like for, and, nor, but, or, yet, or so).
Choice C
Choice C is incorrect. This choice results in a run-on sentence, an error caused when two independent clauses are joined without punctuation or appropriate conjunctions. Since both the clause before the blank ("In 2000…settings") and the clause after the blank ("she…professionals") are independent, a comma would be required in addition to the coordinating conjunction "and."