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.
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. About 1% of the air is still gas composed of the noble gas Argon, Ar.