With the exception of the developer option listed above, which as is suggested by its name, a developers tool.... You don't close apps in Android, that is not how android works.
Android memory management is completely different than in the olden days. There is no "x" box to close a window on an app. The active app on the screen gets priority. After that Android shuts down apps based on a large number of circumstances, mostly having to do with memory. If the app in front needs more memory then it will release the older stuff in order to free up that memory for other uses. In theory memory your not using right now is always available whether it has another program in it or if its empty. Closing apps will not speed up your phone, and will not save on any battery (1 because forcing apps out of memory just makes them have to be reloaded rather than popping back up when you need them and 2 apps dictate when they run, not the user, an app that needs to run in the background will usually wake its self up and run and there is little you can do to stop it.)
As noted in another post, you can clear the recent apps list using the recents menu/button, however it only clears the list, it does NOTHING to clear those apps from memory... remember "available memory" means nothing in Android.
Android doesn't look at an app as "open"
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