Answer :
let's look at the sentences without the relative clauses, this should make it easier:a. Dave Wiens helped design the Olympic mountain bike course in Atlanta, Georgia.
b. Exploring international mountain passes are one of the biker’s favorite hobbies.
c. Wiens’s competitors inspires him to train more aggressively.
d. Knee injuries, neck and back pain, and weakness in the hands develops when poor bike training is used. We see here that in the first sentence "Dave Wiens helped" - and this is correct. Option A is correct. In C it should be "competitors insipire" and in B exploring...is". in D we have a number of things, so it should be "develop"
the correct answer is : Dave Wiens, a Denver native and champion mountain biker, helped design the Olympic mountain bike course in Atlanta, Georgia.