How can the bandwidth of a Yagi antenna be increased?

Increasing the physical diameter (thickness) of the elements on a Yagi antenna reduces the antenna's Q-factor. A lower Q-factor broadens the frequency range over which the antenna maintains an acceptable impedance match and performance, thereby increasing its overall bandwidth. The other options do not increase bandwidth. Decreasing element spacing typically narrows bandwidth and affects gain. Using loading coils electrically lengthens the elements but significantly narrows the bandwidth.