An example of a common ferromagnetic material is

a. plastic.
b. hydrogen.
c. nickel.
d. copper.

C

Explanation:

ferromagnetic in formally means that a substance can be magnetized the way iron would be.

Plastic is a no starter. It cannot be magnetized even in your dreams. Not A

Hydrogen is a gas. That does not eliminate it entirely, but it does lesson the chances. Not B

Copper conducts electricity (well). But it cannot be magnetized. Not D

The answer is Nickel. It is nearly as good at being magnetized as iron and in alloys it is better.

b. a lightning bug, plug of grass, and a worm

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 =