The correct answer is "it's"
When snowboarding, it's wise to wear protective gear that will protect you from injury.
In this case, the sentence is an adverbial subordinate clause.
It is a subordinate clause that has an adverbial function.
Adverbial subordinate clauses are generally introduced by subordinating conjunctions such us before, as soon as, when, etc.
Besides, it is important to mention that adverbial clauses need to be attached to independent clauses so as to make complete sentences.