The correct answer is "neutron".
In fact, a particle is affected by an electric field only if it is electrically charged. The neutron is electrically neutral (its charge is zero), so it does not interact with the electric field. Instead, all the other particles (alpha, which consists of 2 protons and 2 neutrons; beta+, consisting of a positron; beta-, consisting of an electron; and proton) are electrically charged, so they are affected by the electric field.
