A 4x10-6 C charges lies 5 m away from a -6x10-6 C charge.
Is this force attractive or repulsive?

there are no contact forces mentioned on the list of choices that you've provided.

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 =