Consider a system of three harmonic oscillators (SHOs), each with possible energies 0, 1ε, 2ε, 3ε, ... If you are given the total energy, then you know that the sum of all the oscillators' energies is equal to the total energy. One way to compute how many microstates there are is to write down how many states there are if the first oscillator has 0, 1, 2, and 3 ϵ energy, then sum them all together.

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 =

the answer is the first electromagnet (a). this is because it has more coils around the ferromagnetic material. increasing the number of coils increases the surface area in which the metal interacts with the current in the coil. this enables more atoms in the metal to become aligned hence becoming more magnetized.