Of the following needles, which one has the smallest bore, or diameter?

The choice identified as correct, which is the 30 G needle, is accurate because it has the smallest gauge number, indicating a narrower diameter or bore compared to the others listed. In the gauge system used for needles, a higher gauge number corresponds to a smaller bore size. The 30 G needle, therefore, is thinner than the 20 G, 25 G, and 18 G options.

While the measurement of length is relevant when considering needles, the key factor in this question is the gauge, as it directly influences the bore's diameter. In practice, a 30 G needle would be utilized in scenarios where a very fine delivery of medication is required, such as for injections into delicate tissues or for administering very low volumes of fluids.