Both are OK. I understand why there is confusion here. You have probably been taught that there are two kinds of relative clauses: 1. A "defining" or "restrictive" relative clause. This does NOT use a comma and can take either "that" or "which." For example: "There aren't a lot of companies THAT/WHICH are hiring now." This is a defining relative clause because the information is essential -- if you take away the clause, the sentence says "There aren't a lot of companies" and that isn't right. 2. A "non-defining" or "non-restrictive" clause. This is for non-essential information; it uses a comma and can only take WHICH. For example: "New York, which has about 8 million people, is the largest city in the U.S." You can remove the relative clause and the sentence still makes sense. ("New York is the largest city in the U.S.") SO... in your sentence, there is a comma so at first glance it appears to be a non-restrictive clause and can only use "which." Right? BUT please notice two things: * "that" is modifying "techniques," earlier in the sentence, NOT "interruptions." * if you remove the information in the clause ("that are commonly used for managing time"), the sentence no longer has the right meaning. ("There are various techniques,such as delegating to others and reducing interruptions, and when we put these into practice,we will soon notice the difference.") Something is missing when the relative clause is removed. So, for this reason, your sentence actually has a defining / restrictive relative clause and can use "that." It is tricky because the "such as" phrase is inserted between "techniques" and "that." There are various techniques that are commonly used for managing time... There are various techniques ///,such as delegating to others and reducing interruptions, /// that are commonly used for managing time... You have to ignore the "such as" phrase when you analyze the structure of the sentence. I hope this helps!

Either word is fine.

in this case, u just can use "which" for sure. It's relative clause. "Which" and "that" are possible to use in Re.clause but there's comma: "interruptions , that". We can't use that in this case:) Hope it's useful :)