The correct answer is D: 1/2 wavelength. A Yagi's driven element is approximately 1/2 wavelength long. The driven element is typically slightly shorter than a free-space half-wave to account for end effects and to optimize performance in the Yagi configuration. The driven element in a Yagi is the element that's actually fed with RF power. It's typically about 0.47 to 0.49 wavelengths long (slightly shorter than a true half-wave) to account for the effects of the nearby parasitic elements (reflector and directors). The exact length depends on the Yagi design, spacing, and desired performance, but it's approximately half a wavelength. This is a fundamental characteristic of Yagi antennas - the driven element is a half-wave dipole, modified slightly for the Yagi environment.