What is the definition of a common ion as it applies to le chatelier’s principle?

an ion that is found in a wide variety of compounds
an ion that is present in every sample of pure water
an ion that is found dissolved in most aqueous solutions that are widely used in labs
an ion that is present in an equilibrium system and a compound added to the system

Astudent pours 10.0 g of salt into a container of water and observes the amount of time it takes for the salt to dissolve. she then repeats the process using the same amounts of salt and water but this time she slowly stirs the mixture while it is dissolving. the student performs the experiment one more time but this time she stirs the mixture rapidly. the dependent variable in this experiment is: time for salt to dissolve speed of stirring amount of water mass of salt