The shielding constant for ${}_{30}Zn$ is:
A) 25.65
B) 15.65
C)25.35
D) 10.35

Hint: Slater's rule is used to calculate shielding constant.

Formula used- $ = (0.35 \times n) + (0.85 \times m) + (1.00 \times p)$ where n is number of electrons in n shell, m is number of electrons in n-1 shell, p is number of electrons in the remaining inner shells.

Complete step by step answer:
Write the electronic configuration of Zinc
Atomic number of Zinc is 30
Hence its electronic configuration is$1{s^2},2{s^2}2{p^6},3{s^2}3{p^6},3{d^{10}},4{s^2}$
Find the number of electrons in n shell.
In the given case, n=4 and the number of electrons is 2. But we will take only 1 electron.
Find number of electrons in n-1 shell.
There are 18 electrons in the third shell.
Find the number of electrons in the remaining inner shells
The remaining inner shells include the first and second shell. There are 10 electrons in the first and second shell.
Calculate the shielding constant using slater’s rule
Shielding effect =$(0.35 \times 1) + (0.85 \times 18) + (1.00 \times 10) = 25.65$

Option A is correct.

Additional information:
When we move from left to right across a period, in periodic table, the number of electrons increases and hence the strength of shielding increases. Electrons in different orbitals have different tendency to shield. Electrons in s and p orbitals are good shielders whereas electrons in the d and f orbitals are poor shielders. Effective nuclear charge on a particular electron can also be calculated by slater's rule. Effective nuclear charge is the pull of the nucleus on electron or measure of the attraction of an electron towards nucleus. Effective nuclear charge can be calculated by subtracting shielding/ screening constant from atomic number.

Note:
In the atom, each electron is said to experience less than the actual nuclear charge because of shielding or screening constant. It is always caused by the electrons intervening between the nucleus and the valence electron.