Are the following sentences right?
a) The book you lent me is very good.
b) The book which you lent me is very good.
c) The book that you lent me is very good.
When can I leave "which/who/that" out?
The relative clause can (and should) be omitted if it doesn't add meaning, information or clarity to the sentence.
The book you lent me, which I never read before, was very good. The non-restrictive relative clause (introduced by which) adds information that is not essential to the meaning of the sentence.
I lost the book that you lent me. This restrictive relative clause (introduced by that ) is essential to the meaning of thiis sentence.
In your examples (a) is the best sentence. The others are not ungrammatical, just not as good as (a).