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.

productive most commonly means: achieving or producing a significant amount or result.

explanation:

productive can also mean:

producing or giving rise torelating to or engaged in the production of goods, crops, or other commodities.

these depend on how the word productive is used. most likely productive will mean   achieving or producing a significant amount or result.