My answer is
(C). It took me 02:03.
(A) Incorrectly compares "the diet (of the grizzly bear)" with "the Alaskan brown bear and most other members of the bear family"
(B) Looks ok. Keep for now.
(C) Looks ok. Keep for now.
(D) The comparison between plural "diets (of the grizzly bear)" and singular "diet (of the Alaskan brown bear and most other members of the bear family)" is dissonant. Why should we use "diets" for one bear but "diet" for more bears?
Also, "Just" at the very beginning is not justified.
(E) Incorrectly compares "grizzly bears" with "the diets".
Now we have to pick one between (B) and (C).
They are quite similar. Eventually, I pick (C) over (B) for the following three reasons.
(1) (C) is shorter and more natural-reading (by getting rid of "those of"). Shorter is better.
(2) There can be two interpretations of "those of the Alaskan brown bear and most other members of the bear family"
2.1 Those of (the Alaskan brown bear and most other members of the bear family) -> This is the intended meaning
2.2 (those of the Alaskan brown bear) and (most other members of the bear family) -> This would result in nonsensical comparison. Why risk the confusion?
(3) B uses "diets of a grizzly bear" while C uses "a diet". It makes more sense to use "diet" for a grizzly bear.