Suppose you know only the position of an object at instants a and b. is more than one displacement possible? is more than one average velocity possible? is more than one average speed possible?

Is more than one displacement possible?
No, displacement only considers the initial and final position.
Is more than one average velocity possible?
No, the average velocity is defined as displacement per time and since there is only one displacement possible, there is only one average velocity possible as well.
Is more than one average speed possible?
Yes, the average speed considers the total distance traveled and this distance may not be the same as the total displacement.