Which chemical would contribute a common ion if
added to this system?
CaCl2
NaF
H2SO4
K2SO4

Iwould you figure it out but you don't have a picture or in your question as to what the physical property you're looking for is

Idon't mean to be a smart alec, length of atoms' bonds is the simplest way to define it.

Answerthe same as;

She was of necessity running a grass roots target audience was unlettered