What is the estimated effective nuclear charge, z eff, experienced by an electron in a 3p orbital of a chlorine atom? a. -1 b. 5 c. 7 d. 17,

Effective nuclear charge is the charge felt by valence electrons of multi-electron atom taking into consideration the number of shielding electrons that surround the nucleus.
To calculate Z eff for electron we use this formula :
Z eff = Z - S where Z is the number of protons in nucleus and S is the average amount of electron density between the nucleus and electron in consideration
so Z eff = 17 - 10 (core electrons) = 7 
So the correct answer is C