Which is one of the following properties of an element is not variable?

The correct answer is Atomic Mass. 

Key Points

​Atomic Mass:

• The Atomic Mass is defined as the sum of the numbers of Neutrons and Protons in the nucleus of an atom and the Atomic Mass of Hydrogen is 1 amu.
• Atomic Mass = Total No. of Protons + Total No. of Neutrons
• The atomic mass of a compound is constant and it is unique for every element.

Additional Information 

Atomic weight:

• Atomic weight, also known as relative atomic mass, is the average mass of an atom of a particular chemical element, taking into account the natural abundance of its isotopes.
• It is expressed in atomic mass units (amu) and is typically measured with respect to
• the mass of carbon-12, which is assigned a value of exactly 12 amu. 

Valency:

• The outermost electron shell of an atom is called the valence shell.
• The electrons present in the outermost shell of an atom are called valence electrons.
• The valence electron of an atom takes part in a chemical reaction because they have more energy than all the inner electrons.
• The combining capacity of an atom of an element to form a chemical bond is called its valency.
• The valency of an element can be variable as in transition metals, Cu⁺, Cu⁺²

Equivalent Mass:

• Equivalent mass is the mass of a substance that reacts or combines with one mole of hydrogen ions (H⁺) or hydroxide ions (OH^-) in a chemical reaction.
• It is also known as gram equivalent mass or equivalent weight.

Equivalent Mass = Molar Mass/Valency

Since it depends on valency, it is also a variable.