The correct answer is A. simple parabola.
A simple parabola is a curve that is defined by the equation $y=ax^2$, where $a$ is a constant. The graph of a simple parabola is a U-shaped curve that opens upwards if $a>0$ and downwards if $a<0$.
A cubic parabola is a curve that is defined by the equation $y=ax^3$, where $a$ is a constant. The graph of a cubic parabola is a three-dimensional curve that has a single maximum point if $a>0$ and a single minimum point if $a<0$.
A spiral is a curve that winds around a central point in a continuous way. The graph of a spiral is a two-dimensional curve that has no beginning and no end.
A lemniscate is a curve that is defined by the equation $x^2+y^2=4$. The graph of a lemniscate is a four-leafed curve that is symmetrical about the $x$-axis and the $y$-axis.
Of the four shapes, a simple parabola is the most common shape used in valley curves. This is because a simple parabola is a smooth, continuous curve that is easy to generate and control. Additionally, a simple parabola can be used to create a variety of different valley shapes, from shallow valleys to deep valleys.