The correct answers are C. untouched and D. pristine.
The sentence is structured to set up a contrast. The first part, "the dense brush can make for tough going," describes a negative aspect of the park. This is contrasted with the second part, which describes a positive aspect that makes the effort worthwhile: a chance to "behold fauna few others ever have."

This suggests the wilderness is in a state that is largely undisturbed by human presence.
- C. untouched: This means not handled, treated, or dealt with; not changed or damaged. This fits the idea that the wilderness is in a natural, undisturbed state.
- D. pristine: This means in its original condition; unspoiled. This is a perfect synonym for "untouched" in this context and also describes a natural, undisturbed environment.

The other options do not fit the context:
- A. empty: While an untouched wilderness might feel empty, the sentence specifically mentions the presence of "fauna," which contradicts this word.
- B. endangered: This refers to species at risk of extinction. While the park might contain endangered species, the blank is describing the state of the wilderness itself, not the animals within it.
- E. secluded: This means not seen or visited by many people. While the park is secluded, "untouched" and "pristine" better capture the specific reason why it's secluded, which is the lack of human interaction.
- F. imperilled: This means to put at risk of being harmed or destroyed. This is the opposite of the idea that the wilderness is in a natural, unspoiled state.