**Group and Valence Electrons:**

In the periodic table, elements are arranged in groups based on their similar chemical properties. A group is a vertical column in the periodic table. Each group is labeled with a number and sometimes a letter.

Valence electrons are the electrons in the outermost energy level or shell of an atom. These electrons are involved in chemical bonding and determining the reactivity of an element. The number of valence electrons dictates an element's chemical behavior, including its ability to form bonds and react with other elements.

**Explanation:**

Elements in the same group have the same number of valence electrons. This is because elements in the same group have similar electron configurations, meaning they have the same number of electrons in their outermost energy level. The outermost energy level is also known as the valence shell.

For example, elements in Group 1 (also known as the alkali metals) have one valence electron, while elements in Group 2 (the alkaline earth metals) have two valence electrons. Similarly, elements in Group 17 (the halogens) have seven valence electrons, and elements in Group 18 (the noble gases) have eight valence electrons (except helium, which has two valence electrons).

Since the number of valence electrons determines an element's chemical behavior, elements in the same group exhibit similar chemical properties. They tend to have similar reactivity and form similar types of chemical bonds.

Having the same number of valence electrons also means that elements in the same group are likely to have similar outermost electron configurations. For example, all the elements in Group 1 have an electron configuration of ns1, where "n" represents the principal energy level. This similarity in electron configuration further contributes to the similar chemical properties of elements in the same group.

Therefore, the correct answer is option 'A': Elements in the same group have the same number of valence electrons.