Which graph best represents a function with a range of all real numbers greater than or equal to -6?

Four graphs labeled A, B, C, and D showing parabolas on coordinate planes; graph A shows a parabola opening upwards with vertex at (-3, 0), graph B shows a parabola opening downwards with vertex at (-3, -8), graph C shows a parabola opening upwards with vertex at (1, -6), and graph D shows a parabola opening downwards with vertex at (-3, -1).