Answer: Not determine their exact position.
Explanation: In the electron cloud model, electrons are said to be moving so fast that a person cannot determine their exact position.
This phenomenon goes well off with the Heisnberg Uncertainity Principle (HUP)which states that the momentum (Δm) and position (Δx) of an obejct cannot be determined simultaneously at the same time.
The mathematical relation of HUP is -
Δx Δm ≥ h/ 4π
where h = Planck's Constant