Which of the following quantities will be zero for alpha particles at the point of closest approach to the gold atom, in Rutherford's scattering of alpha particles by a gold foil?

As the alpha particle approaches nucleus , the electrostatic repulsive force due to the nucleus increases and kinetic energy of alpha particle goes on converting into electrostatic potential energy. At the distance of closest approach, the whole of kinetic energy is converted into electrostatic potential energy. Hence, the kinetic energy is zero at the distance of closest approach.