1) Explanation

Let's break down the sentence to understand the context and the meaning required for the blank:
- "While the path leading to the lake was no longer open to the public, the trampled undergrowth was a $\qquad$ the plethora of hikers who had passed through the formerly pristine forest over the years."
- "While the path...was no longer open to the public": This establishes a current state of closure.
- "the trampled undergrowth": This is a key effect or result. Undergrowth becomes trampled due to heavy foot traffic.
- "the plethora of hikers who had passed through the formerly pristine forest over the years": This is the cause. A "plethora" means an excess, indicating many hikers. "Formerly pristine" emphasizes that the forest was once untouched, implying the trampling is a negative change.
- Therefore, the blank needs a word that indicates the trampled undergrowth is a result or effect of the many hikers.

Let's evaluate the given options:
- A. deference to: Humble submission and respect. This is irrelevant to the cause-and-effect relationship described.
- B. repercussion of: An unintended consequence occurring sometime after an event or action, especially an unwelcome one. This fits perfectly, as trampled undergrowth is an unwelcome consequence of many hikers.
- C. precursor to: A person or thing that comes before another of the same kind; a forerunner. This indicates something that comes before and leads to something else, not a result.
- D. consequence of: A result or effect of an action or condition. This is a direct and strong synonym for "repercussion of" and perfectly fits the cause-and-effect relationship.
- E. harbinger of: A person or thing that announces or signals the approach of another; a forerunner. Similar to "precursor," this indicates something that comes before.
- F. souvenir from: A thing that is kept as a reminder of a person, place, or event. While the undergrowth might be a "reminder," "souvenir" implies something taken or kept, and doesn't capture the direct cause-and-effect of the trampling.

Conclusion:

The sentence clearly describes a cause-and-effect relationship where the trampled undergrowth is a direct result of the hikers' activity. Both "repercussion of" and "consequence of" accurately convey this meaning.

The final answer is $\(\mathrm{B}, \mathrm{D}\)$.