SelectElementDeselectAll Method WebDriver
Clear all selected entries. This is only valid when the SELECT supports multiple selections.

Namespace: OpenQA.Selenium.Support.UI
Assembly: WebDriver.Support (in WebDriver.Support.dll) Version: 3.1.0
Syntax

public void DeselectAll()
Exceptions

ExceptionCondition
WebDriverExceptionThrown when attempting to deselect all options from a SELECT that does not support multiple selections.
See Also

Reference