Which of the sources listed below produces the best monochromatic...

A laser produces light that is highly coherent, directional, and monochromatic, meaning it consists of a single wavelength or color.

A bulb and a candle emit light that contains a continuous spectrum of many wavelengths, making them polychromatic sources.

A mercury tube emits light consisting of several discrete wavelengths, so it is not strictly monochromatic.

Therefore, a laser is the best source of monochromatic light among the given options.