Change the italicized group of words into a gerund phrase. then write the complete sentence. 'to hike through canyons' is strenuous activity.

"To hike through canyons" will be converted into a gerund phrase by changing the verb "to hike" into a verb that end in -ing.

So, To hike through canyons is strenuous activity becomes Hiking through canyons is a strenuous activity.

Gerund phrases starts with a gerund, or a word that ends in -ing. It is followed by other modifiers or objects. Gerund phrases function as nouns. So, they can be subjects, subject complements, or objects in the sentence.

gerund phrase - Hiking through canyons 
gerund - hiking 
serves as subject of the sentence.

It has to be probably c