Compared to an electron, which particle has a charge that is equal in magnitude but opposite in sign?
(1) an alpha particle (2) a neutron
(3) a beta particle (4) a proton

I think the correct answer is proton since protons have a charge of +1 while electrons have a charge of -1.  However, since beta particles are used to name a type of nuclear decay and can refer to either a positron or electron depending on the circumstances, your teacher might want you to put beta particle since positrons are basically electrons but that have a positive charge.
 I hope this helps.  Let me know if anything is unclear.