Answer:
.
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.