The correct answer is D) "Although I have a lot to say, I'm not sure what you have already heard at this point."
The dependant clause in the sentence is "Although I have a lot to say" since it does not have meaning on its own.
The independant clause is "I'm not sure what you have already heard at this point." because it can stand on its own and has complete meaning.
Dependant clauses, also called subordinating clauses, start with a subordinating conjuction and it may function as an adverb, adjective or noun in a sentence. In this case, the subordinating conjunction is "although" and it introduces an adverbial clause.
