Which of the following increases the resonant frequency of a dipole antenna?

There is a direct inverse relationship between the resonant frequency of an antenna and its physical length. Because radio waves travel at the speed of light, higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. To make a dipole antenna resonate at a higher frequency, it must be sized for that frequency's shorter wavelength. Therefore, you must physically shorten the antenna. Lengthening the antenna, adding coils (inductors), or adding capacitive loading will all make the antenna electrically longer, which lowers its resonant frequency rather than raising it.