Answer
The light bulb must be placed at 1 inch above the vertex.
Work Step by Step
As per the given statement , we have $y=1$
and Diameter = 4 inches. This implies that Radius: $2x=4 \implies x=2$
Now, consider $x^2=4py$
This implies that $(2)^2=(4p)(1) \implies p=1$
Hence, the light bulb must be placed at 1 inch above the vertex.
