What Does the Abbreviation SOF Indicate on an Insurance Claim

What does the abbreviation SOF indicate on an insurance claim?

A) The physician will accept any payment by the insurance company as payment in full.
B) The office has verified that the patient is eligible for benefits for the services billed on the claim.
C) The office maintains a document signed by the patient that authorizes submission insurance claims.
D) The physician promises that all information on the claim form is correct to the best of his or her knowledge.