The castle has a mass of 500 grams and a volume of 250 mL. How much water does it displace? 500 grams or 250 mL ?

250 mL

Explanation:

If the density is more than that of water (in this case it is) it will sink to the bottom of the container holding water (if it fits).

That being the case, the amount of water displaced = the volume of the castle.

250 mL

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 =