An atom has 40 electrons and 46 neutrons. how many protons does it have?

The correct statement is that their identity changes in the reaction B and not in reaction A.

Explanation:

It is the atomic number on which the identification of an atom solely depends upon. The number of protons presents within an atom of an element is the atomic number of that element. The arrangement of the element takes place in the periodic table in the order of increasing atomic number.  

As mentioned in the reaction A, the electrons are gained simply by the atoms of an element and are getting converted into a negative ion, there is no change taking place in the identity of the atom. On the other hand, in reaction B, the identity of the atom is getting changed as protons are lost by the atom, and the identity of the atom only changes when the number of protons within the atom changes.