The correct option is B Although
A subordinate clause (it may not be the most eloquent oratory) is dependent on the main clause (Harry's speech as best man at his friend's wedding was heartfelt) for its full meaning. They're hence, of unequal rank in a sentence. When these two are combined, a subordinating conjunction is used. The underlined word 'whereas' is a coordinating conjunction used when both the clauses are independent on their own and are of equal rank or importance in a sentence. Since the given sentence does not have clauses of equal rank, one of them is dependent on the other, 'whereas' is incorrect and hence, option A is incorrect. Similarly, 'additionally' is a coordinating conjunction used when the clauses are both independent, and the conjunction merely adds the two. Hence, option D is incorrect. The given clauses are contrast or opposite in meaning, Harry's speech was not eloquent, in spite of that, it was heartfelt. 'Although' is a clause used for contrast-related sentences, hence option C is correct. 'If' is a conjunction used when the subordinate clause is a condition that determines the action in the main clause. Since the subordinate clause here is not a cause, but a contrast, option B is incorrect.