The smallest unit that retains the properties of an element is a(n)

The given blank can be filled with the atom.  

An atom is the minutest component of a matter, which exhibits the characteristics of a chemical element. Every liquid, solid, plasma, and gas comprises ionized or neutral atoms.  

The atoms are extremely small, the usual sizes are about 100 picometers. Each atom contains a nucleus and one or more electrons combined with the nucleus. The nucleus is formed of one or more protons and usually a same number of neutrons.  

the relative standard reduction potential of the half-cell in which reduction occurs; more positive than the other half-cell. reduction is a chemical process in which electrons are added to an atom or an ion; it always occurs accompanied by oxidation of the reducing agent.

correct answer is a. and d.