I believe the correct answer would be D. The complex sentence would be "The panther crept into a cave because it was afraid." A complex sentence would be a sentence that contains an independent and an dependent clause. The independent clause would be "The panther crept into a cave" while the dependent would be "because it was afraid".
Here's a corrected version: "Have you ever ran in a relay race? If you can imagine a relay race, then you'll understand how
<span> the Pony Express worked.</span>" Only one main punctuation error.
The answer is D.
Because of the wording of the sentence, the base word enthusiastic must be used as an adverb, not an adjective.