Motorcyclists should inspect their helmets and check the motorcycle's tires before: D. they ride.
Subject-verb agreement entails that singular or plural subjects should pair with singular or plural nouns or pronouns.
In the sentence provided above, the subject is -Motorcyclists which is in the plural form.
It is therefore expected that the corresponding pronoun will be in the plural form. This corresponding pronoun is -they.
So, option D is correct.
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