Which of the following situations would cause the greatest increase in the
motion of molecules in a system?

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 =

answer a fan shaped mass of sediment deposited at the base of a mountain front do to an abrupt change in stream gradient;

365.25 earth days are in the year of a light year