There exists one real zero when the vertex lies on the x-axis, from that we conclude that the correct option is A.
The real zeros of a quadratic function are the values of x at which the graph of the quadratic function intercepts the x-axis.
If we want only one real zero, then we must have only one intercept. And this only can happen if the vertex of the quadratic function lies on the x-axis.
As you can see, the only graph where that happens is the one in option A, so we conclude that option A is the correct one.
If you want to learn more about quadratic functions.
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