that or which
In which sentence? Please read this first (from the American Heritage Dictionary):

The standard rule requires that that should be used only to introduce a restrictive (or defining) relative clause, which identifies the entity being talked about; in this use it should never be preceded by a comma. Thus, in the sentence The house that Jack built has been torn down, the clause that Jack built is a restrictive clause identifying the specific house that was torn down. Similarly, in I am looking for a book that is easy to read, the restrictive clause that is easy to read tells what kind of book is desired.

A related rule stipulates that which should be used with nonrestrictive (or nondefining) clauses, which give additional information about an entity that has already been identified in the context; in this use, which is always preceded by a comma. Thus, we say The students in Chemistry 101 have been complaining about the textbook, which (not that) is hard to follow.The clause which is hard to follow is nonrestrictive in that it does not indicate which text is being complained about; even if the clause were omitted, we would know that the phrase the textbook refers to the text in Chemistry 101.

Some grammarians extend the rule and insist that, just as that should be used only in restrictive clauses, which should be used only in nonrestrictive clauses. Thus, they suggest that we should avoid sentences such as I need a book which will tell me all about city gardening, where the restrictive clause which will tell me all about city gardening indicates which sort of book is needed. But this extension of the rule is far from universally accepted, and the use of which with restrictive clauses is common.

Furthermore, since that cannot be used with clauses introduced by a preposition (whether or not restrictive), which is used with both clauses when such a clause is joined by and or or to another that does not begin with a preposition, as in It is a philosophy in which the common man may find solace and which many have found reason to praise. Such constructions are often considered cumbersome, however, and it may be best to recast the sentence completely to avoid the problem.

That is often omitted in a relative clause when the subject of the clause is different from the word that the clause refers to. Thus, we may say either the book that I was reading or the book I was reading. In addition, that is commonly omitted before other kinds of subordinate clauses, as in I think we should try again, where that would precede we. These constructions omitting that are entirely idiomatic, even in more formal contexts.