What type of right mixture is a perfect exposure a result of?

The key element is the amount of light you allow into your camera when taking a picture. The more light, the brighter the picture. The aperture setting controls how much light is let in. But this may not in itself be 'perfect' for not all pictures require utter brightness.

Settings must be adjusted for the effect you want. to decrease brightness you would be increasing your shutter speed, upping your aperture (higher number, smaller hole), and/or reducing your ISO. The best pictures have the right Aperture + shutter speed + film speed - i.e. the last option in the list of possible answers.