Groups are an important method of classifying because a) their elements have the same atomic mass b) they categorize only the most important elements c) they validate the organizing principals of the periods d) their elements have very similar properties and exhibit a clear trend

                Option-D, "their elements have very similar properties and exhibit a clear trend" is the correct answer.

                         In periodic table the elements are arranged in tabular form with respect to their atomic masses, atomic numbers, electronic configurations and chemical properties. It is called periodic because the properties of elements repeats periodically.

                         Furthermore, the rows from left to right also called Periods basically shows the metallic and non metallic characters of elements. \those on the left are metals and those at the rights are non metals.

                        The columns from top to bottom are called as Groups. Groups contain elements of same chemical properties.

Group I elements are called as Alkali Metals. They all have one valence electron and hence, loose one electron to form the corresponding cation. i.e.

                                           M    →    M⁺¹  +  1 e⁻

Group VII elements are called Halogens, they all are non metals and tend to gain one electron to form the corresponding halide ion. i.e.

                                           X  +  1 e⁻    →    X⁻¹

Is there options? If not, then your answer is going to be cells!