Question

what do all atoms of the same element have in common?
a total mass
b number of protons
c number of neutrons
d size of nucleus

Explanation:

Atoms of the same element are defined by having the same number of protons. Protons determine the element's identity. For example, all hydrogen atoms have 1 proton. Isotopes of an element (which have the same element name) can have different numbers of neutrons (so mass can vary as mass = protons + neutrons). The size of the nucleus isn't a defining common - factor for all atoms of an element as it can be influenced by various nuclear - level factors.

Answer:

B. number of protons