Aperture Priority A

Explanation: Aperture priority means that you choose an aperture (f-stop) and the D5100 will set a corresponding exposure time.
The aperture can be selected with the command dial on the back.

Recommendation: This is an alternative to mode P for easy choice of a specific aperture.

But you can get the same results in mode P by rotating the command dial on the back to shift the combination of aperture and exposure time.