Option (iii) is correct because the frequency of a sound wave is defined as the number of density oscillations at a fixed point per unit time. An oscillation consists of a compression and a rarefaction, so increasing the frequency directly increases the number of compressions per second. Conversely, increasing the frequency will decrease the wavelength and the time period (since time period is the time taken for one complete oscillation). The speed of sound in a medium depends on the properties of the medium, not on the frequency, so it remains unchanged.