There are a few different ways you can answer this, depending on what terms your book uses and which treaty they think came first.
At any rate, the first arms reduction pact involving naval power came out of the negotiations at the Washington Naval Conference of 1921. This was the first arms reduction conference of any sort in history.
There were three major treaties that came out of this conference. Of these, however, I think that the one that you are looking for is the one called the "Five-Power Treaty" or the "Washington Naval Treaty." These are the same treaty and I assume that is what you are supposed to give as the answer.