Final answer:
Aylmer is resuming his studies of the 'C) Human body' after having previously investigated various natural phenomena.
Step-by-step explanation:
Aylmer, as a character in the passage provided, is deeply interested in the natural sciences and the wonders of the world. He has made discoveries about volcanoes, fountains, mines, and other natural wonders.
It is clear from the context that Aylmer is resuming his studies of the human body. This is reflected in the text where it states that he had previously studied the human frame and aimed to understand the process by which nature assimilates influences to create and sustain human beings.
His interest lies in physiological truths that are related to his plans for treating Georgiana. Therefore, the correct answer to fill in the blank based on the provided passage is 'C) Human body'.