A. "That" introduces a restrictive clause; "which" introduces a nonrestrictive clause. To put it differently, a clause beginning with "which" is going to add some incidental information to the sentence. A clause beginning with "that" is going to add something necessary for the clarity of the sentence. "Which" clauses are usually set off by commas.
For example:
The training that you need for this career is available only at XYZ Institute.
The training, which everyone on our staff has completed, is very thorough.