Answer
The maximum possible height above the Earth's surface for the bottom of the thundercloud is 300 meters.
Work Step by Step
We can find the distance above the Earth's surface:
$E~d = \Delta V$
$d = \frac{\Delta V}{E}$
$d = \frac{1.00\times 10^8~V}{3.33\times 10^5~V/m}$
$d = 300~m$
The maximum possible height above the Earth's surface for the bottom of the thundercloud is 300 meters.
