edexcel physics alevel question

answer is A idk y

apparently its to do with all kinetic energy being converted into potential energy but isnt potential measured in Volts? and the equations for (electric potential) V=kQ/r, has ONLY one charge, it assumes that the the charge being investigated has to be 1C, when can you suddenly do kQQ/r=V, thats not a thing is it????

someone explain pleasee

There are no non-conservative forces acting on the alpha particle so kinetic energy (KE) + potential energy (PE) of alpha particle is conserved.
So for alpha particle change in KE + change in PE = 0
change in KE = 0 - (initial KE)
change in PE = kQ1Q2/r - 0
so r = kQ1Q2/(initial KE) which is A

kQ/r is potential energy per unit positive charge due to a point charge Q. Q = (+) 79 X 1.6 X 10^-19 for gold nucleus and this needs to be multiplied by the charge of the alpha particle which is (+) 2 X 1.6 X 10^-19 to find the potential energy of the alpha particle at a distance r from the gold nucleus.

p.d. measured in volts is work done per unit charge, when this is multiplied by charge of alpha particle you get units of energy J.

is this a hard question or am i just dumb