'that' is the most appropriate word to use here.

both are strictly equally appropriate. in fact there's a third option identically valid which is to not put anything in the blank area. in voiced language, most of the time this kind of optional word tends to be skipped by most speakers (i believe). (a fourth option is actually to put both, "that which"; i believe it is seen as an improper form although a lot of people use it in specific circumstances. but i think best to avoid the off topic ^^') anyways, the difference between both words lies in that "which" is seen as (more or less) slightly more formal, pertaining to a (slightly) more "elevated" speech style. but the fact is nowadays, whether you use one or the other you won't be seen as sloppy or snobby, unless you meet an old-school english speaker that might assert the dire necessity to use "which" over "that" whenever possible (or an antl-old-school speaker that will call your language outdated (in slightly less diplomatic words, usually)). other than that, i'd honestly advise for choosing for yourself the one of both (of the three (four?) possibilities) that which "feels" better to you. the choice won't matter at all anyways. you can even alternate depending on the situation, most people do. there are a lot of synonyms in english, typically one of a pair being seen as *slightly* more formal, but without a real strong difference between the two. in a lot of other cases, you can just choose the one that sounds the better. of course, all this depends if you're talking to an old friend or if you're writing a letter to apply for a new job. insofar as it is appropriate, i'd say the choice matters even less when you consider that most of the time, one's language style will most certainly evolve towards emulating the ones of the english-speaking people you'll meet and talk to, or read, extensively, quite alike the accent.

that