Eugen Goldstein in 1886 discovered the presence of positively charged rays in a gas discharge tube. He called these rays as canal rays. This experiment led to the discovery of protons. 

Electrons are negatively charged whereas protons are positively charged. It was found that electrons are attracted by a positive charge and protons are attracted by a negative charge.

Bohr's Model of the atom included the idea(s) that:

The electron can have only certain energies, including a lowest-level ground state, electrons emit energy as light when they move to the lower energy orbits and electrons absorb energy by moving to the higher energy orbits.

An element X is represented as ${}_Z^{A}X$, where 'Z' is the atomic number and 'A' is the mass number. So, for the given options -
$$\begin{array}{ccc}\text{Element}& \text{Atomic no.}& \text{Atomic mass no.}\\ \text{A}& \text{12}& \text{25}\\ \text{B}& \text{18}& \text{40}\end{array}$$

1. First orbit or K-shell (n=1)  = 
2x$1^2$ = 2
2. Second orbit or L-shell (n=2)  =
2x$2^2$ = 8
and so on.

When alpha particles are sent through a thin metal foil only one out of ten thousand rebounded. Rutherford concluded that positively charged particles are concentrated at the centre of the atom. At the time of the experiment, the atom was thought to be analogous to plum pudding (as proposed by J.J. Thomson), with the negative charges (the plums) found throughout a positive sphere (the pudding). If the plum pudding model were correct, the positive "pudding", being more spread out than in the current model of a concentrated nucleus, would not be able to exert such large Coulombic forces, and the alpha particles should only be deflected by small angles as they pass through.