According to the AWS shared responsibility model, when a company uses Amazon EC2 instances to host their applications or databases, they are responsible for the security of the guest operating system. This includes tasks such as applying security patches, managing user access, and ensuring that the software is configured securely. AWS is responsible for the security of the underlying infrastructure, including the EC2 hypervisor, the physical network connectivity of the host server, and the uptime service level agreement (SLA) for the EC2 instances. Therefore, the correct answer is B, 'Security patching of the guest operating system', as it is the company's responsibility to manage this aspect of the EC2 instances.