Which one is appropriate?

Have you ever had trouble deciding between that and which? They can feel interchangeable, and it isn’t easy to know how to use them. Every writer has habits, things they tend to repeat. These habits can form a cohesive style, but habits can also weigh down writing. If a writer has the habit of using that when they should use which, it might be one of those things that readers don’t notice, but editors will. Even the most casual reader will probably notice when a sentence sounds “off” if that and which are used incorrectly. Being correct never interferes with style, so it benefits any writer to understand the difference.

That is general; which is reserved for specific things.

Mary Norris explains how to tell the difference.

Also, Jessica, our proofreader, adds:

Don’t forget to add a comma before¬†which!¬†