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

The resonant frequency of an antenna is inversely proportional to its physical length. Because radio waves with higher frequencies have shorter wavelengths, an antenna must be physically shorter to resonate at a higher frequency. Therefore, if you need to tune an antenna so its resonant frequency goes up, the correct action is to cut or shorten its elements. Conversely, if you wanted to lower the resonant frequency, you would lengthen the antenna. The other options all have the opposite effect of shortening. Lengthening the wire, inserting series coils (inductive loading), or adding capacitive loading to the ends will all electrically or physically lengthen the antenna, thereby lowering its resonant frequency.