Instataneous velocity is the velocity at a particular instant of time in a particular direction. For a given time interval, the instantaneous velocity may or may not vary at different instants of time. Hence, it may or may not have a single value during the motion.
Similarly, instantaneous speed also may or may not have a single value.
Average velocity is single valued for a given time interval. It is given as a ratio of total displacement by total time taken. Hence, it is dependent on initial and final positions only. Thus, it is single valued.
Average speed is the ratio of total distance travelled to the total time taken. Hence, it depends on the path taken to cover the distance in a particular interval of time. Thus, average speed is not single valued.
Hence, out of the four quantities, average velocity is single valued.