Which type of filter essentially only transmits a single wavelength?

An interference filter is specifically designed to transmit light of a very narrow wavelength range while reflecting or absorbing other wavelengths. This ability is due to the interference of light waves, which occurs in these filters as a result of multiple layers of materials with varying refractive indices. The constructive and destructive interference of light at specific wavelengths allows for the selective passage of those wavelengths, effectively making the interference filter capable of transmitting a single wavelength with high precision.

In contrast, a narrow band filter, while also designed to transmit a limited band of wavelengths, typically has a broader range than an interference filter and may not be as selective. A broad band filter allows a wider range of wavelengths to pass through without significant attenuation. A long-pass filter, on the other hand, transmits all wavelengths longer than a specified cut-off wavelength, allowing for a range of wavelengths above that point rather than focusing strictly on one specific wavelength. This distinct functionality of interference filters makes them the correct answer for the question regarding the transmission of a single wavelength.