Which of the following best describes an orbital?
a) a space in an atom where an electron is most likely to be found
b) space where electrons are unlikely to be found in an atom
c) space which may contain electrons, protons, and/or neutrons
d) small, walled spheres that contain electrons
e) a single space within an atom that contains all electrons of that atom

a) a space in an atom where an electron is most likely to be found

Explanation:

Atomic orbital is the mathematical function which describes wave-like behavior of the electrons present in the atom.

It is used to calculate probability of finding the electron of the atom in any  region around nucleus of the atom. Atomic orbital is the physical region or a three dimensional space where the probability of finding the electron is more than 90% which also means that the space where an electron is most likely to be found. They are boundless space and have definite energy.

Option A best describes atomic orbital.