Which property of the element is not variable?

The ability of an element to combine with other elements to form chemical compounds or molecules is known as valency. The quantity of hydrogen atoms an atom combines with helps us gauge its affinity or combining power. As an illustration, the combination of two hydrogen atoms and oxygen results in a water molecule with a valency of 2.
Atomic mass is the weight of an element's atom. Each element has a distinct atomic mass.
The mass of one equivalent, or the mass of a particular material that will combine with or displace a specific quantity of another substance, is known as the equivalent weight. The molar mass can be used to compute equivalent weight with ease.

Valency of the elements can be more than one. Equivalent mass is the molecular mass by valency. So, both valency and equivalent mass is variable. Atomic mass is the constant quantity. 

Hence, the correct answer is B.