Answer:
By showing his ad to people searching for related information about helmets.
Explanation:
Although other options seem helpful, the best thing Google can do is to show his ad when people search for the helmets. The reason is that when people search something on the internet, mostly it is the time when they want to buy that stuff. On the other situations like seeing your ad while they search for other stuff, they may not pay enough attention for your product (That is because they are searching something else).
