According to natural selection, individuals that tend to survive are those that have which of the following? . . a. the ability to quickly produce the most offspring. . b. the largest and strongest body structures. . c. the genetic variations that are best suited for their environment. . d. the fastest running speeds.

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 =