Question

Which sentence reflects the correct usage of a plural possessive noun?

Olympic snowboarders determination drives them through the rigorous' training required to remain competitive in the sport.
Olympic snowboarders's determination drives them through the rigorous training required to remain competitive in the sport.
Olympic snowboarders' determination drives them through the rigorous training required to remain competitive in the sport.
Olympic snowboarders determination drives them through the rigorous's training required to remain competitive in the sport.

Answer 1

Olympic snowboarders' determination drives them through the rigorous training required to remain competitive in the sport.

Answer 2

The sentence that reflects the correct usage of a plural possessive noun is the following one: Olympic snowboarders' determination drives them through the rigorous training required to remain competitive in the sport.

The possessive of most plural nouns is formed by adding an apostrophe only (snowboarders' determination).  That is why that sentence is the correct option.

However, there are plural nouns which do not end in -s. The possessive of such nouns is formed by adding an apostrophe and an -s to the noun. For instance, "people's power".