How do i find the resultant acceleration

Let denote the North-facing force, and the South-facing force. Let's also take North-facing vectors as having positive direction.

The resultant force on the body is

Then we have

where is the resultant acceleraton vector. So

It looks like you assumed gravity was involved here, but that doesn't seem to be the case, at least not to me. We're given two forces, and only these two forces as far as I can tell, so you shouldn't assume there are any others acting on the body.

the answer is b i think

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 =