Which of these particles is the smallest? A. a proton

B. a nucleus

C. an electron

D. an atom

C

Explanation:

so basically, we can eliminate some answer choices right away.

an atom (answer choice D) is made up of all the other answer choices, so that is the largest (not the answer).

a nucleus (answer choice B) is made up of protons (answer choice A) and neutrons which means it is not the smallest either (not the answer).

now, we are left with a proton and an electron. electrons are much smaller than protons, so they are usually not included in an element's atomic mass. therefore, an electron is the smallest out of these answer choices.