Representative elements are grouped according to which of the following?

a.

valence electrons

c.

valence shell

b.

representative electrons

d.

physical state at room temperature

B. Representative electrons

Firstly it is best to describe the shape and structure of an atom, an atom's system and structure is like of the solar planet, the heliocentric view where the sun is the neutrons and protons and the planets are the electrons. Moreover, the electrons are outside these neutrons and protons surrounding the entire system of its atomic structure. Its movement is like of a planet. 

An energy level also known as electron shell, or just shell is a location around an atom where electrons with a specific  energy value,  or quantum number may be found.

Use a magnet to separate iron, evaporate water, i'm not sure how to separate the other two i'm sorry but good luck