The periodic table is a vital tool in chemistry and physics, classifying elements according to their properties and atomic structures. We can validate each option to see if each statement is true regarding the long form of the periodic table:

Option A: "It reflects the sequence of filling the electrons in the order of sub-energy levels s, p, d, and f"

True. The long form of the periodic table is designed based on the electron configuration of elements, where elements are arranged in blocks corresponding to the type of atomic orbital (s, p, d, or f) being filled with electrons. The filling order reflects the Aufbau principle, which predicts the order in which orbitals are filled: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p. This arrangement allows chemists and physicists to understand and predict the electronic structure of the atoms.

Option B: "It helps to predict the stable valency states of the elements"

True. The periodic table provides information on the valence electrons of elements, which are the electrons that participate in chemical bonding. The elements in the same group (column) have similar valence electron configurations, leading to similar chemical behaviors, including valency. For example, all elements in Group 1 have one electron in their outermost shell and typically exhibit a +1 oxidation state in compounds.

Option C: "It reflects trends in physical and chemical properties of the elements"

True. The periodic table systematically arranges elements in rows (periods) and columns (groups) based on increasing atomic number and similar electronic configurations, respectively. This setup reflects and predicts trends in both physical (such as atomic radius, ionization energy, and electronegativity) and chemical properties (such as reactivity and common oxidation states) across periods and down groups.

Option D: "It helps to predict the relative ionicity of the bond between any two elements"

Generally, less accurate compared with other statements. While the periodic table can suggest trends in electronegativity (which affects ionic character when different elements form compounds), predicting the relative ionicity of bonds between any two elements requires more specific information about their electronegativity differences, which may not be directly inferred from the periodic table alone. Rather, it requires specific calculations or additional data aside from just the position in the periodic table.

In conclusion, Options A, B, and C are true statements regarding the long form of the periodic table, whereas Option D, while somewhat supported by trends indicated in the periodic table, is not fully accurate without further information or context. Thus, D is less directly supported by the layout of the periodic table alone.