Explanation
Correct answer: D.
Choice D is the best answer. The convention being tested is punctuation between two main clauses. “Aibonito is well known for its lush greenery” and “this distinction has earned it the fitting nickname…” are both complete main clauses; a semicolon correctly joins them while signaling a close logical connection.
Why the other choices are wrong
Choice A
Choice A is incorrect because it results in a run-on sentence. The two main clauses are fused without punctuation and/or a conjunction.
Choice B
Choice B is incorrect because it results in a comma splice. Without a conjunction following it, a comma can’t be used in this way to join two main clauses.
Choice C
Choice C is incorrect. When coordinating two longer main clauses such as these, it’s conventional to use a comma before the coordinating conjunction.