In an experiment, two groups of mice—one whose diet included ginkgo extract and one that had a normal diet—were taught to navigate a maze.
Conclusion (however)
The superior ability of the ginkgo mice to navigate the maze on day 2 might not be directly attributable to a memory boost from the ginkgo.
Evidence (studies have found .. and .. )
Ginkgo reduces stress in mice, and lowering very high stress levels is known to improve recall.
Evaluate
This question ends up testing a really small language shift that I did not catch the first time I tried the problem, so I'm writing this explanation as though I am that person, since we can still put ourselves in a position to understand the correct answer without noticing it.
Because we're interpreting the results of an experiment, which featured a Curious Comparison, we can use our causal mindset.
Curious Fact ... why were the ginkgo mice more likely to remember how to navigate the maze on day 2 than were the normal mice?
One Explanation ... ginkgo extract directly boosts memory
Author's Explanation ... ginkgo, as we know, reduces stress, and lowering stress is known to improve memory.
Goal
Since this is Weaken, we'd either want to suggest an Alternate Explanation for why the ginkgo mice were better able to remember the maze (we could return to the notion that ginkgo directly boosted memory, but some 3rd explanation could also weaken)
or
lower the plausibility of the Author's Explanation that the ginkgo mice were better able to remember the maze because the ginkgo had lowered their stress.