25.
Your Answer is
Correct Answer is A
Explanation
The adverbial clause is omitted. This question examines the omission of adverbial clauses guided by Whether. Whether he is rich or poor, I'll marry him. After being omitted, it is Be he rich or poor, I'll marry him. Therefore, according to the meaning of this sentence "Although his books deal with many melancholy themes, whether slaves system, loss of harvest, or misunderstanding between friends..." is the omission of the whether conditional sentence, after reduction it should be—whether it is slavery, the loss of a harvest, or a misunderstanding between friends—.