Frequency is defined as the total number of complete waves in an unit time. A complete wave consists of a complete cycle of the amplitudes of waves.
In all the figures, we see transverse waves. They have the mean position denoted by a straight line, and the waves do oscillate up and down to complete a full cycle. The most number of waves in a given time has the highest frequency. If we count the full number of waves in the particular given time, Wave A has 5.75 complete waves, wave B has 3.5 complete waves, wave C has 2 complete waves and the wave D has 2.75 complete waves. So the wave A has highest number of complete oscillations in a given time. So wave A has highest frequency.
