Which process is used to test whether a solution containing a salt is saturated? A) add a crystal of the same salt to the solution, B) cool the...

A saturated solution means that at that specific
temperature you cannot dissolve more of the solute into the solvent.  In your case, if
the solution is saturated then adding a crystal of the same salt, and stirring, would
give you the best chance of determining of the solution is saturated or not.  It is
important not only to add the crystal but also stir because some salts dissolve slowly. 
If you just add the crystal and see it sink to the bottom that may indicate the solution
is saturated or it may mean it takes time for the salt to
dissolve.

Cooling the solution does not prove that the
original solution was saturated.  If it was saturated, cooling it may result in a
supersaturated solution rather than crystallization of some of the solute.  And if some
salt does crystallize that would be expected even if the original solution was not
saturated since most salts are less soluble at lower
temperatures.

Heating would prove nothing and stirring
vigorously would be pointless if there is no solute undissolved.