This is why E cannot be the answer:
E says that (E) Gourmet cooks, fast-food cooks, and cooks who are health food enthusiasts are all big eaters
However, the stimulus says "Some good cooks are gourmet cooks ... Some good cooks can be characterized as fast-food cooks...Other good cooks are health food enthusiasts"
Note that apart from these three group of cooks, some other cooks (that are not mentionned in the stimulus) may exist!! the text never convey the contrary!!
so GOOD COOK is the sum of gourmet cooks, fast-food cooks , health food enthusiasts AND POSSIBLY SOME OTHER TYPE OF (good) COOKS !!!
Now, when the stimulus say "But even though not all good cooks are big eaters...",
it may be, as (E) says, that "Gourmet cooks, fast-food cooks, and cooks who are health food enthusiasts are all big eaters.", since other types of cooks could be qualified as "Good cooks" AND, in this case, one would conclude that those good cook category (that are not mentionned in the text) are those who are not big eaters, and only them.
Hence, (E) could be true and cannot be the answer!
HTH