Just because he's an insect in both doesn't make that the theme. The theme of a story is it's underlying message, the big picture of a story, what the author wants you to take away from the story not just what they story says it's about on the surface.
I believe the answer is either C. The disconnection between mind and body, or D. The sense of duty toward one's family.
It is definitely NOT A or E, those are very basic answers. I've seen a lot on here who "know" the answer is A, but those people are obviously not thinking hard enough. I'm sorry, but it's true. This is really starting to bother me.
I think the answer your looking for is D. The sense of duty toward ones family. Although he is suffering a mind-body conflict here, it is mainly because of his family in these passages.
Somewhere else in the book, most likely near the beginning of his change, he was actually happy to be like this, relieved that he was finally free of his responsibilities. However, that changes when he realizes that their sense of peace and comfort will end with him like this, he begins to worry and tries not to think about it. "But what now, if all this peace and wealth and comfort should come to a horrible and frightening end? That was something that Gregor did not want to think about too much"
In the second passage here, he is trying to remain calm and hopeful that he might change back so that his family deal with his new existence. He feels like he is imposing, once he was there provider and now he is this thing that is taking up space and comfort in there lives. "in worries and vague hopes which, however, always led to the same conclusion: for the time being he must remain calm, he must show patience and the greatest consideration so that his family could bear the unpleasantness that he, in his present condition, was forced to impose on them."
You don't haft to take my word for it, but PLEASE do your research before just accepting answers on here! If they do not provide you with specific examples and explanations, do not believe them wholeheartedly! Even if they say they're on plato.