What is the test for excusable neglect under section 473 of the California Civil Code?

To establish excusable neglect, a litigant must demonstrate conduct that " 'might have been the act of a reasonably prudent person under the same circumstances.' " (Hearn v. Howard (2009) 177 Cal.App.4th 1193, 1206.) " 'The inadvertence contemplated by the statute does not mean mere inadvertence in the abstract. If it is wholly inexcusable it does not justify relief. [Citations.] It is the duty of every party desiring to resist an action or to participate in a judicial proceeding to take timely and adequate steps to retain counsel or to act in his own person to avoid an undesirable judgment. Unless in arranging for his defense he shows that he has exercised such reasonable diligence as a man of ordinary prudence usually bestows upon important business his motion for relief under section 473 will be denied. [Citation.] Courts neither act as guardians for incompetent parties nor for those who are grossly careless of their own affairs. . . . The only occasion for the application of section 473 is where a party is unexpectedly placed in a situation to his injury without fault or negligence of his own and against which ordinary prudence could not have guarded.' " (Ibid.)