Which factor principally limits horizontal television resolution?

The correct answer is related to the concept of "bandpass," which specifically refers to the frequency range that a system can adequately transmit. In the context of horizontal television resolution, bandpass limits the amount of detail that can be resolved in an image. This is because the resolution is determined by how well a system can reproduce high-frequency components of the image, which correspond to fine detail.

A higher bandpass allows for better transmission of these high frequencies and, consequently, better horizontal resolution. If the bandpass is limited, it results in loss of detail due to the inability to transmit the higher frequency signals that correspond to the finer lines or patterns in the image.

In terms of the other factors, frame rate and field rate primarily affect the motion display and the smoothness of video playback rather than the resolution itself. Lines/frame refers to the vertical resolution in a frame and speaks to how many horizontal lines can be displayed, but it does not limit the horizontal resolution directly in terms of clarity of detail represented. Thus, bandpass is the key factor that limits horizontal television resolution.