This argument is a bit light on obvious indicator words – the only one we have is "yet" marking the transition from the context to (what turns out to be) the conclusion.

In the absence of indicator words, the approach is to look at each claim and ask "do any other claims support this one?" Here's the answer key, so to speak:

Context: Most pleasure cyclists don't ride until spring/summer.
Support: Most bicycles are purchased in the spring.
Support: When shoppers are ready to buy, they've already made up their minds.
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Conclusion: It's probably more effective to advertise bicycles earlier than that.

The two claims marked as support work together to explain why earlier advertising is a good strategy. By contrast, for example, if we ask "Why should I believe most bicycles are purchased in the spring?", neither of the other claims explains why that's true.

Wrong answers in main conclusion questions tend to phrase the conclusion inaccurately, accurately state premises, or inaccurately state a premise.