The first three choices: a, b and c can be considered reconstruction except the last one which is letter d. I'm not really certain what reconstruction is, but judging from the patterns of the first three choices, reconstruction could mean that an inference is made after a part of an event has proved that event to be true.
Protons and neutrons are located in the nucleus, a dense central core in the middle of the atom, while the electrons are located outside the nucleus.
Explanation:
it is given that, the linear charge density of a charge, 
Firstly, we can define the electric field for a small element and then integrate for the whole. The very small electric field is given by :
..........(1)
The linear charge density is given by :


Integrating equation (1) from x = x₀ to x = infinity



Hence, this is the required solution.
A best describes magnetism