The correct answer is B: Beta or hairpin match. The Beta or hairpin matching system for Yagi antennas requires the driven element to be insulated from the boom. This is because the beta/hairpin match works with a capacitively reactive feed point, which requires the element to be electrically isolated. The beta (or hairpin) match uses a U-shaped conductor (the 'hairpin') connected across the driven element. This match works when the driven element is slightly shorter than resonance (capacitive). The element must be insulated from the boom so it can have the proper reactive impedance. If the element were connected to the boom (grounded), the impedance characteristics would be different, and the beta match wouldn't work properly. Other matching systems like gamma match can work with the element connected to the boom.