The correct answer is Option B - the IT manager, the Chief Information Security Officer (CISO), and the affected business unit leader. These are the primary stakeholders who need to be aware of significant vulnerabilities affecting critical systems. The IT manager will be involved in the technical remediation process, the CISO is responsible for the overall security posture and risk management, and the affected business unit leader needs to be aware of potential impacts on business operations. Option A is incorrect because, typically, a customer service representative does not play a direct role in the management or mitigation of vulnerabilities. Option C is incorrect as external auditors are generally not considered immediate stakeholders for vulnerability reporting within the firm, although they might be informed in line with regulatory requirements. Option D is incorrect because product vendors do not need to be the primary point of communication for internal vulnerability management unless the vulnerability originates from their product, in which case they are notified as part of the remediation process.