Choice "D" is correct. A firm's failure to establish or comply with an appropriate system of quality control does not necessarily imply that the firm has failed to follow professional standards on individual engagements.
Choice "a" is incorrect. Quality control standards relate to the conduct of a firm's entire practice whereas professional standards such as GAAS relate to the conduct of an individual engagement.
Choice "b" is incorrect. The adoption of an effective system of quality control standards is conducive to complying with professional standards on individual engagements.
Choice "c" is incorrect. Deficiencies in or noncompliance with a firm's quality control standards do not necessarily indicate a lack of compliance with professional standards for any one specific engagement.