Choice "D" is correct. A WebTrust engagement is an attestation engagement in which the CPA determines whether the client's web site meets defined criteria relating to transaction integrity, information protection, and disclosure of business practices. Attestation engagements should be performed in accordance with Statements on Standards for Attestation Engagements (SSAEs).
Choice "a" is incorrect. Statements on Assurance Standards is a distractor; there is no such standard.
Choice "c" is incorrect. Consulting services provided by CPAs range from advice on accounting-related matters to a wide range of services involving diverse technical disciplines. Consulting services performed by CPAs should be performed in accordance with Statements on Standards for Consulting Services (SSCSs).
Choice "b" is incorrect. Audits (generally of financial statements) performed by CPAs should be performed in accordance with Statements on Auditing Standards (SASs).