Has any one can me with this one. i got it wrong the first time i know it’s not b.

Smokey !  Mass doesn't change ! You're supposed to know that.  If the ball has 5 kg of mass on Earth, then it has 5 kg of mass on the moon, on Pluto, in orbit, in free fall, on a roller coaster, in the grocery store, in church, in the shower, or in my sock drawer.  Mass doesn't change ! WEIGHT depends on gravity, so it can change. MASS doesn't and can't.

electric field is a function of 1/r^2

yes, the force per unit charge for a point charge is inverse square law.

it is given that trillions of microbes make up to 1% to 3% of the body mass.

let's take average of this percentage: 2%

consider the average mass of the human body as 70 kg.

then, trillions of microbes would have mass  

therefore, 100 trillion micro-organisms would weigh:

this is an approximate value and could change depending on the mass of the human body and percentage of the microbes present in it.

average mass per microorganism =