Answer:
.
Step-by-step explanation:
Let , , and be constants, and let . The equation represents a parabola in a plane with vertex at .
For example, for , , , and .
A parabola is entirely above the -axis only if this parabola opens upwards, with the vertex above the -axis.
The parabola opens upwards if and only if the leading coefficient is positive: .
For the vertex to be above the -axis, the -coordinate of that point, , must be strictly positive. Thus, .
Among the choices:
- does not meet the requirements. Since , this parabola would open downwards, not upwards as required.
- does not meet the requirements. Since and is negative, the vertex of this parabola would be below the -axis.
- meet both requirements: and .
- (for which ) would touch the -axis at its vertex.