Light travels fastest in a vacuum, but among these options, it travels fastest in the substance with the lowest refractive index. The speed of light in a medium is given by $v=\frac{c}{n}$, where $c$ is the speed in vacuum and $n$ is the refractive index. Benzene has the lowest refractive index (approximately 1.50) compared to water (1.33), kerosene (~1.44), and turpentine (~1.47). Therefore, light travels fastest in benzene.

Final answer: benzene