Answer:
D). The panther crept into a cave because it was afraid.
Step-by-step explanation:
A complex sentence is illustrated as a sentence that consists of at least one dependent or subordinate clause beginning with subordinate conjunction along with a principal or independent clause where the former is dependent on the latter to accomplish its meaning.
As per the question, option D exemplifies a complex sentence as it comprises of an independent clause 'The panther crept into a cave' which functions to accomplish the meaning of subordinate clause 'because it was afraid' followed by it beginning with the subordinate conjunction 'because'. The other options either exemplify a simple sentence(option A) or a compound sentence(options B and C due to the lack of subordinate conjunction).