Estimate the total charge q on the cloud when the breakdown of the surrounding air is reached

First, you must breakdown electric field:

E = 9x10^9 q / R^2

where R is the radius of the cloud

q is the required charge.

So this will come up to a solution of:

q = E R^2 / 9x10^9

= 3 x10^6 x (500) ^2 / 9 x10^9

= 83.33 C.

The answer is 83.33 C.