I want to show Timezone Drop down at registration time, Joomla provide any settings show drop down either code?
No, by default it does not. User can set timezone in his/her "Profile" page only after registration.
You can use Easy Profile extension to extend the users manager. This is probably the best of its kind.
I'm not sure if it will have a timezone selection already built in, but if not you can make custom fields and drop downs.