The smallest needle size among the given options is gauge 25, as the higher the gauge number, the smaller the needle.
The correct answer to the given question is option c.
The sizes of needles are indicated by their gauge numbers. A key point to remember is that the larger the gauge number, the smaller the needle.
With this system, a gauge of 18 means the needle is larger than those with a higher gauge number like 21, 25, or 15.
Therefore, the smallest needle from the provided options would be the one with the highest gauge number, which is gauge 25 (option c).