The selection of particular item in the DropDownList can be restricted by using the client side event name called “select”.
Initialize the DropDownList as follows
When selecting the items in the DropDownList, the select event will be triggered, in that check whether the corresponding item is selected or not.
If the particular element is selected, then set the value of the DropDownList as “null”, so that the item cannot be selected as well as if you had set a water mark in the DropDownList, that water mark will be shown.
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