Answer:
Participle.
Step-by-step explanation:
When a word that is usually a noun is used as an adjective, that word is called a participle. (These words often, but do not always, end in -ing; they can also end in -ed, or other irregular endings.)
The word "hiking" is used as a participle. "Hike" is a verb, and clearly denotes an action. However, when used in this first sentence, you can see that "hiking" actually helps describe the "trails," so it is used as an adjective.
The word "frightening" is also used as a participle. "Frighten" is a verb, but in the context of the second sentence, "frightening" is used to describe "hike."