The correct answer is option 'B', which states that the valency of an element determines its chemical behavior. Valency refers to the ability of an element to combine with other elements to form compounds. It is determined by the number of valence electrons present in an atom of an element.

The valence electrons are the electrons present in the outermost energy level or shell of an atom. These electrons play a crucial role in chemical reactions as they are involved in the formation of chemical bonds. The number of valence electrons determines how many other atoms an element can bond with and the type of bonds it can form.

Here is a detailed explanation of why valency is the property that determines the chemical behavior of an element:

1. Definition of Valency:
Valency is a measure of an element's combining capacity and is determined by the number of valence electrons. Valence electrons are the electrons in the outermost energy level of an atom.

2. Formation of Chemical Bonds:
Chemical bonds are formed when atoms share, gain, or lose electrons. The number of valence electrons in an atom determines how it will interact with other atoms to form compounds. Atoms tend to gain, lose, or share electrons to achieve a stable electron configuration, usually by attaining a full outer shell of electrons.

3. Types of Bonds:
The valency of an element determines the type of chemical bonds it can form. Elements with a valency of 1 tend to lose one electron to form a positive ion (cation). Elements with a valency of 2 tend to lose two electrons or gain two electrons to form positive or negative ions. Elements with a valency of 3 tend to gain three electrons or lose three electrons to form positive or negative ions, and so on.

4. Reactivity:
The valency of an element also determines its reactivity. Elements with a low valency, such as those in Group 1 of the periodic table, have a strong tendency to lose electrons and form positive ions. These elements are highly reactive. On the other hand, elements with a high valency, such as those in Group 17 (halogens), have a strong tendency to gain electrons and form negative ions. These elements are also highly reactive.

In conclusion, the valency of an element determines its chemical behavior because it determines the element's ability to bond with other elements and the type of bonds it can form. The valence electrons play a crucial role in chemical reactions and the formation of compounds.