The formula for the circumference of a circle is mandatory stuff: it's πD (or π times the diameter). We've been told the circumference is 18π, so the diameter must be 18. Thus, A is the answer.
(Side note: the testmakers are hoping you get the diameter and the radius mixed up here. It's not intellectually challenging to distinguish between the two, but it's an easy mistake to make no matter how good you are at math, so always be careful with radii and diameters. My recommendation is that you never deal with diameters, and if you're given a diameter, always just change it to a radius. You're less likely to accidentally use one in place of the other this way. But in this particular problem, I broke my own rule because they literally gave us the diameter and asked for it so it seems foolish to convert it to a radius and back again.)
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