The activity most likely to increase internal audit quality is increasing audit staff training. ISACA’s resources on audit capability and internal audit development consistently emphasize building knowledge and skills as a way to improve audit effectiveness and quality. Training strengthens auditors’ technical competence, professional judgment, and ability to identify emerging and atypical risks.

Option A is correct because better-trained auditors are more capable of planning effectively, gathering reliable evidence, assessing controls, and producing useful findings. ISACA training resources emphasize foundational and practical audit knowledge, while reskilling guidance highlights the need for internal audit to improve capability in response to changing risks.

Option B is not the best answer because outsourcing may or may not improve quality; it depends on the provider and governance over the arrangement. It is not inherently more effective than developing internal capability.

Option C is incorrect because increasing the number of planned audits can easily reduce quality if resources are stretched thinner. More volume does not automatically mean better audit work.

Option D can support continuous improvement, but client surveys are an indirect feedback tool. They do not improve technical audit quality as directly as stronger auditor training and competency.

Therefore, A is the best answer because improving auditor competence through training is the most direct and reliable way to increase internal audit quality.

References (Official ISACA):

ISACA, IT Audit Fundamentals Certificate — emphasizes foundational audit concepts and practical application of audit principles.

ISACA Journal, Reskilling Internal Audit — highlights the need for internal audit to improve capabilities to identify emerging and atypical risk.

ISACA Now Blog, Internal Auditing With an Agile Scrum Approach — collaboration and feedback can improve quality and relevance, but competence remains foundational.