A CPA assists a taxpayer in tax planning regarding a transaction that meets the definition of a tax shelter as defined in the Internal Revenue Code. Under the AICPA Statements on Standards for Tax Services, the CPA should inform the taxpayer of the penalty risks unless the transaction, at the minimum, meets which of the following standards for being sustained if challenged?a. Substantial authority.b. Realistic possibility.c. More likely than not.d. Not frivolous.