The greatest threat posed by quantum computing is its potential to break widely used cryptographic algorithms (A), particularly public-key encryption schemes that rely on mathematical problems vulnerable to quantum algorithms. This directly threatens data confidentiality and long-term data protection. The other options describe secondary or non-security impacts. CISM recognizes cryptographic resilience as a core component of risk management and emphasizes the need for organizations to monitor emerging technologies that could invalidate existing controls. Quantum computing's impact on encryption represents a systemic risk to trust, privacy, and secure communications.
References: ISACA CISM Review Manual (Risk management-emerging technologies and cryptographic risk); CISM Exam Content Outline (Domain 1).