Quantum algorithms, such as Shor's algorithm, can factor large prime numbers exponentially faster than classical computers, threatening the security of RSA and elliptic-curve cryptography. Similarly, Grover's algorithm reduces the effective strength of symmetric key algorithms by half, requiring larger key sizes.
While post-quantum cryptography is being developed, current algorithms may become obsolete once practical quantum computers exist. Options B, C, and D are incorrect because quantum does not inherently improve encryption, nor is training the key issue-it is the fundamental breakage of cryptographic assumptions.
References (ISACA): ISACA Journal - Cryptographic Risks and Quantum Computing; CISA Review Manual, Cryptography.