This is tricky, I was also stuck between C and E for a while.
I think it is pretty clear that Passage A discusses both answer choices.
C: "They are enforced with sanctions, including simple badmouthing and refusals to work with an offending comedian." (Line 29)
E: "The answer to this question is that, in stand-up comedy, social norms substitute for intellectual property law." (line 25)
The question is, which one is missing from passage B? We know what the social norms are among chefs. But, unlike passage A, we have no idea what would happen to a chef who breaks these norms and copies a recipe. Passage B never explains what the consequences would be. This is why C is the right answer.
I would argue that E is discussed in passage B. What is the impact of social norms on creative output (recipes and techniques)? I am able to answer this question. The social norms in the restaurant industry protect a chef's original recipe. A colleague cannot share a chef's recipe with others. A chef must be given credit as the creator of an original recipe. Although the words "creative output" are not explicitly used, it is safe to assume that this term applies to recipes.
