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sra gowthamsra gowtham 

 how to prevent users from deleting task if it is closed using validation rule

Steven NsubugaSteven Nsubuga
You cannot prevent record deletion using a validation rule. Consider a trigger.
trigger preventTaskDeletion on Task (before delete) {
    for(Task tk : trigger.old){
		if (tk.Status == 'Closed') {
			tk.addError('This Task Cannot be deleted');
		}
    }
}