He felt no ...... as he plunged the knife into her back.

In this context, Option A: "qualms" is the correct word. It means a feeling of doubt or uncertainty about the morality or rightness of an action. The sentence suggests that the person didn't hesitate or feel any doubts about their actions, making "qualms" the most appropriate choice. The other options do not convey the same sense of moral hesitation.

Option B: scruple
While "scruple" is related to hesitation or doubt about the morality of an action, it doesn't fit well in the context of the sentence. "He felt no scruple" sounds less natural and is less commonly used in this context compared to "qualms."

Option C: conscience
"Conscience" refers to a person's inner sense of what is right and wrong, and it doesn't directly convey the absence of hesitation or doubt. Saying "He felt no conscience" wouldn't be the typical way to express that someone didn't hesitate or have doubts about their actions.

Option D: morals
"Morals" refers to principles of right and wrong behavior, and it doesn't directly convey the idea of hesitation or doubt. Saying "He felt no morals" would suggest something different, such as a lack of moral values, rather than the absence of hesitation or doubt.

In the given sentence, "qualms" is the most appropriate choice because it specifically addresses the lack of hesitation or doubt about the morality of the action, making it the best fit in the context.