Please see the “Control/Task/Task Action Link” control.
Per the tag documentation:
A control that provides an output link that inherits most of the properties of the output Link control that you can use inside Task Notifications to initiate a Reply To email action on the task. When this link is used in the Task Notification, it creates a link inside the email body. Clicking this link creates a pre-filled reply email message, including To: email address, Subject, and body. A user can send it by using a native mail client like Outlook or Outlook Express.
Sending this email automatically performs a configured action on the task, for example, updating business data, like accept or not-accept, or completing a task, as configured on the control.