How electric field can be less than force per unit test charge?

I have been going through some physics objective questions and struck with this one Q:A negatively charged metallic ball is supported on a rigid insulating stand. We wish to measure the electric field E at a point P in the same horizontal level as that of the metallic ball. To do so, we put a positive charge q and measure F/q. The electric field at the point P is
a) =F/q
b) <(F/q)
c) >(F/q)
d) none of these.
Guess what?. I chose (a) and its wrong. (b) is correct. I am squeezing my brain. How can that be correct.? Please give detailed explanation.

Answer

The field strength is only $F/Q$ if the test charge is infinitesimally small so it cannot affect the metal ball.

For example, suppose the metal ball is uncharged in which case the fortce should be zero. A positive charge will polarise the ball and create an attractive force even though the charge on the ball is zero. In the case of a charged ball the positive charge will polarise the ball and increase the atttractive force.