This is a Critical Reasoning question that asks for a premise that weakens the effectiveness of the proposed solution.

Argument Analysis
1. Problem: The silvertail fox population has seen a marked decrease.
2. Suspected Cause (Correlation): The decrease occurred during the same time as the influx of hunters (due to eased sanctions), and the fox is a popular target.
3. Conclusion/Solution: Reinstate strict sanctions against hunting to prevent further population decline.

The Goal: Find a reason why stopping the hunting (the proposed solution) will not prevent the fox population from falling further. This means finding an alternative, ongoing cause for the decline.

Evaluating the Options
- A. The population of rabbits has surged ever since the hunting sanctions in Deersdale County were lifted.
- A surge in rabbits (the fox's food source) should support the fox population. If the fox population is still declining, it strongly suggests the hunters are the cause. This strengthens the conclusion.
- B. The silvertail fox population varies greatly throughout the year, especially during winter when prey becomes scarce.
- This suggests that the "marked decrease" observed in the last several months might be a natural, cyclical occurrence (e.g., they measured the population at its usual seasonal low point) rather than a direct result of hunting. If the decrease is natural, placing strict sanctions will not prevent the natural variation and is not the appropriate solution. This weakens the conclusion's effectiveness.
- C. The local authorities are expecting even more hunters in the coming year to arrive to the park.
- This suggests the problem is likely to get worse due to hunting, strongly supporting the need for stricter sanctions.
- D. The silvertail fox had been experiencing a population surge shortly before the state sanctions against hunting were eased.
- This is irrelevant background. The decline that followed the surge could still be entirely due to hunting. (Does not weaken).
- E. The grey wolf, a large predator that competes with the silvertail fox over Deersdale Preserve's rabbit population, has seen its numbers decrease since the arrival of the hunters.

- A decrease in a competitor (the grey wolf) should theoretically benefit the silvertail fox population. Since the fox population is still declining despite reduced competition, this makes hunting seem even more likely to be the primary cause. This strengthens the conclusion.

The only option that provides an alternative explanation for the decline (natural variation) that would persist even if hunting stopped is $\mathbf{B}$.

The correct choice is $\(\mathbf{B}\)$.