Which statement is true about nuclear fusion?
a. it is caused by the same process that causes nuclear fission.
b. it does not occur naturally in the solar system.
c. it has very low activation energy.
d. it produces nearly all the elements that are heavier than helium.

The true statement is d.

Explanation:

Nuclear fusion is the process in which two lighter nuclei are combined to form one heavy nucleus along with the huge amount of energy. This reaction takes place in the Sun.

In the Sun, there is a fusion of the hydrogen nuclei to form helium nuclei.

Nuclear fusion produces more energy than nuclear fission.

Nuclear fission is the process in which one heavy nucleus splits to give lighter nuclei along with the tremendous amount of the energy. Atom is the example of the nuclear fusion.  

Therefore, the true statement is d.    

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 =