The correct answer is D: Very low impedance. A 1/4-wavelength transmission line open at the far end presents very low impedance to an RF generator. A quarter-wavelength line transforms an open (infinite ohms) to a short (very low impedance) at the input. Transmission lines transform impedances based on length. An open 1/4-wavelength line inverts the impedance - an open at the far end becomes a short (very low impedance) at the input. This is the opposite of a shorted 1/4-wavelength line, which transforms a short to an open. This property is useful for creating low-impedance points, shorting circuits at specific frequencies, or creating RF shorts. The 1/4-wavelength transformation is fundamental to many matching and filtering applications.