Answer:
D) Knowing that Mrs. Mallard was afflicted with a heart trouble, great care was taken to break to her as gently as possible the news of her husband's death.
Step-by-step explanation:
The best revision of the sentence includes a comma to separate the present participle clause Knowing that Mrs. Mallard was afflicted with a heart trouble from the next clause.
The rest of the choices are incorrect because option A has no commas at all, sentence B contains unnecessary commas, and option C separates the suject from the verb in the participle clause.