The correct option is B Though
A subordinate clause ("the people respected him") depends on the main clause ("the emperor was tyrannical") for its full meaning. To combine these two in a sentence, a subordinating conjunction is required. In the given sentence, the subordinate clause expresses the contrast, or opposite of the main clause. Hence, a conjunction that refers to a contrast-related sentence is required. The underlined word 'because' is used for cause-related sentences, whereas the given sentence is not contrast-related. Hence 'because' needs to be replaced, and option D is incorrect. 'Though' is used for contrast related sentences, in the given statement it expresses the alternatives "the emperor was tyrannical" and "people respected him". Because a tyrant would not gain respect, but fear from the people, hence these ideas are contrast, and 'though' is appropriate, so option B is correct. 'Since' is used for cause-related sentences, whereas the given statement expresses contrast, hence option A is incorrect. 'Besides' means 'in addition to', and is used to refer to an additional clause, apart from the main clause, and these clauses are merely connected together, without affecting each other. Since the sentence is contrast-related and does not have merely an additional clause, option C is incorrect.