Solution:
The vertical line test is used to determine if a graph is a function or not.
According to the vertical line test, a function has only one dependent variable (output) for each unique input (independent variable).
Using the vertical line test, when a vertical line cuts the graph not more than once, the graph is a function.
Thus, from the given options,
An equation with an exponent:
The vertical line cuts the graph more than once. Thus, the graph is not a function.
A circle with a center (0, 0):
The vertical line cuts the graph more than once. Thus, the graph is not a function.
A parabola that opens to the right:
The vertical line cuts the graph more than once. Thus, the graph is not a function.
A parabola that opens upwards:
The vertical line cuts the graph not more than once. Thus, the graph is a function.
Hence, a parabola that open s upwards is a function.
The correct option is D.