As always, we start by reading the question first. It tells us that we are looking for an assumption made in drawing the conclusion.

What's the conclusion? It is: innovation in bicycle technology is limited by what authorities will accept as standard for purposes of competition in bicycle races.

Why? Because The only cyclists seriously interested in innovation and willing to pay for it are bicycle racers.

Did you notice any new, surprising words in the conclusion? The premise talks about bicycle racers, but the conclusion talks about standard for purposes of competition. Is there a connection between those two things? The author assumes that there is.

(C) is the best answer. It is the one that connects the idea in the premise (bicycle racers) to the idea in the conclusion (standard for purposes of competition).

But imagine that you didn't see that. Is there another way to get to the answer? Sure. Use the why test.

C: Innovation in bicycle technology is limited by what authorities will accept as standard for purposes of competition in bicycle races.
Why?
A: Bicycle racers do not generate a strong demand for innovations that fall outside what is officially recognized as standard for purposes of competition, and
P: The only cyclists seriously interested in innovation and willing to pay for it are bicycle racers.

Does that make sense? I think it does. But what if you're still not convinced? Try negating the conclusion to see whether the argument fails.

C: Innovation in bicycle technology is limited by what authorities will accept as standard for purposes of competition in bicycle races.
Why?
A: Bicycle racers generate a strong demand for innovations that fall outside what is officially recognized as standard for purposes of competition, and
P: The only cyclists seriously interested in innovation and willing to pay for it are bicycle racers.

Does this argument make sense? No, it does not. If racers generate a strong demand for innovations that fall outside the official standard, then the conclusion is false.