Answer:
The third option is correct, Caesium (Cs).
Step-by-step explanation:
The effective nuclear charge, Zeff, is a measure of the net positive charge experienced by an electron in an atom. It is defined as the difference between the atomic number (Z) and the shielding constant (S).
The shielding constant accounts for the electron repulsion caused by other electrons in lower energy levels, which effectively "shield" an electron from the full charge of the nucleus.
The general trend in the periodic table is that Zeff increases across a period (from left to right) and decreases down a group (from top to bottom). This is because, as you move from left to right, you're adding protons to the nucleus which increase the positive charge, while the shielding effect remains relatively constant. As you move down a group, you're adding additional electron shells which increase the shielding effect and therefore decrease the Zeff.
Given the options:
Calcium (Ca) is an alkaline earth metal in Period 4.
Chlorine (Cl) is a halogen in Period 3.
Caesium (Cs) is an alkali metal in Period 6.
Germanium (Ge) is a metalloid in Period 4.
Out of these elements, Caesium (Cs) is furthest down the periodic table and therefore would have the most electron shells, leading to a greater shielding effect. Thus, Caesium (Cs) will have the lowest effective nuclear charge, Zeff, among the given elements.