Who is authorized to sign the Articles of Incorporation i California?

The incorporator is to sign the Articles of Incorporation, as some companies that are newly formed won't even decide on officers until after the business entity is formed.

The attorney, the person incorporating the corp. It depends how far along you are into the incorporation process. If you don't have an attorney I suggest you consult with one. I would also recommend a CPA.

Any adult can do this. Often it is the attorney. Often it is a person who will become an officer or director or officer. The organizer resigns after the first meeting of shareholder/directors or manager/members.