Not necessary, because the argument is concerned with what happens to the fish after release. How much the hatchery operators are spending and whether they’re making sustainable economic decisions when they supply visually stimulating environments has nothing to do with the effects of being raised in an experimental hatchery.

Not necessary, because the author’s conclusion concerns only the comparative survival rates of fish after they are released. The environment can play a significant role in survival rate, but one kind of fish can still have a higher chance of survival than another kind of fish.

Necessary, because if this were not true — if NO fish raised in traditional hatcheriess die because they are too timid in foraging for food — then the fact the experimental-hatchery fish are bolder in trying new types of food would not provide support for their having a higher chance of survival. So in order to rely on boldness in trying new food types as a premise for the conclusion, the author must assume (C).

The author doesn’t have to think the released fish need to eat “many different types” of food to survive. Although the author does assume that the released fish will be more likely to survive if they are bolder in trying new types of food, it’s possible that once a fish finds a new type of food to survive on the wild, it can stick to that one source of food. Boldness is important for being able to find new food outside the hatchery environment; but that doesn’t imply fish need to eat a lot of different kinds of food to survive.

Not necessary, because the comment about visually stimulating environments related to the experimental hatchery conditions, and it was provided as a reason that the fish raised there are bolder. But that line of reasoning doesn’t require the author to think the wild environment is also visually stimulating. A fish can benefit in the wild from being bolder, even if the wild doesn’t present the same type of visual stimulation.