ANS:A - atomic number.

The elements in the periodic table are arranged in order of their atomic number. The atomic number of an element represents the number of protons found in the nucleus of its atoms. It uniquely identifies each element because no two elements have the same number of protons. Dmitri Mendeleev's periodic table was initially arranged by increasing atomic mass, but later revisions led to the current arrangement based on atomic number. This change became necessary as it better aligned with the properties and behaviors of the elements, particularly when considering isotopes and electronic configurations. So, the correct answer is atomic number. The atomic number of an element is a fundamental property that uniquely identifies each type of atom. It is denoted by the symbol 𝑍Z and represents the number of protons found in the nucleus of an atom of that element. Here's a breakdown of the key points about atomic number:

1. Definition: The atomic number of an element is the number of protons in the nucleus of each atom of that element. Since the number of protons in an atom determines its chemical identity, the atomic number uniquely identifies each element.
2. Significance: The atomic number is a fundamental characteristic of an element. It determines the element's position in the periodic table and its chemical properties. Elements with the same atomic number belong to the same chemical element, regardless of their mass or isotopic composition.
3. Role in Periodic Table: The modern periodic table is arranged in order of increasing atomic number. This arrangement reflects the periodic trends in chemical properties as well as similarities in the electronic configurations of elements.
4. Relation to Mass Number: The mass number of an atom, denoted by 𝐴A, is the total number of protons and neutrons in its nucleus. While the mass number may vary among atoms of the same element due to different numbers of neutrons (isotopes), the atomic number remains constant for all atoms of a given element.
5. Determining Electron Configuration: The atomic number also determines the number of electrons in a neutral atom. In a neutral atom, the number of electrons equals the number of protons (atomic number). The arrangement of these electrons in energy levels or shells around the nucleus follows specific rules based on the atomic number.