Explanation From HCIA-Security documents:

PKI is designed to solve trust and security problems in digital communications by using certificates, public/private keys, and signature mechanisms. It helps parties verify each other’s identities through certificate-based authentication, so B is a classic problem PKI addresses. PKI also enables secure key distribution and supports security protocols that provide confidentiality and integrity , which helps protect data against eavesdropping and tampering during transmission, so C is also within PKI’s scope. In addition, PKI supports digital signatures and non-repudiation-style evidence, which can replace or supplement paper receipts by creating verifiable transaction records, making D something PKI can help with in electronic transactions.

However, network congestion and service degradation due to heavy traffic (A) is a performance and capacity issue. PKI does not reduce traffic load, increase bandwidth, optimize routing, or fix server resource bottlenecks. Those problems are handled through network engineering measures such as QoS, scaling, load balancing, and capacity planning—not by PKI. Therefore, the correct answer is A .