Of the following elements, the one with the smallest atom is : Rb, K, Li, or F. Why?

The smallest atom you have listed would be lithium (Li).  Lithium is the third smallest atom listed on the periodic table of the elements.  It has an atomic number of three, meaning it has three protons in it's nucleus.  It has a mass number of six point nine atomic mass units, meaning it also has three neutrons in it's nucleus.  Lithium has two electrons orbiting the nucleus in it's first energy level, and one third electron in the outer energy level.

The other elements, in order of increasing atomic number would be fluorine (F), atomic number nine, potassium (K), atomic number nineteen, and rubidium, atomic number thirty-seven.  With increasing atomic number, it is reasonable to expect increasing mass number as well.  The corresponding mass numbers for those elements, in increasing mass number order, are fluorine, nineteen, potassium, thirty-nine point one, and rubidium, eighty-five point five.