Answer:
A. a parabola that opens down.
Step-by-step explanation:
First we can eliminate two answers, b and d, since parabalas that open left or right use the y variable.
Now the equation of a standard parabola is x². This means that there are no negative y values, and therefore it cannot open down, it is up.
With a NEGATIVE coefficient, the graph is flipped over the x axis, therefore making it open down.
Graphical proof is attached for further understanding, positive a is in red, negative a is in blue.