Which statement has the correct syntax to define an $\_$init$\_$ method $($initialize method$)?$

def init$($self$,$ a$,$ b$)$:

def init$\_\_($self$,$ a$,$ b$)$:

def $\_\_$init$\_\_($self$,$ a$,$ b$)$:

def $\_\_$init$($self$,$ a$,$ b$)$: