Answers:
C. Lack of vendor support.
Explanation:
In formation technology, network administrators and engineers experience network security thread due to a large number of vulnerabilities from poorly configured security protocols and end of life of network devices, which results from the outdated nature of a device leading to the end of continual upgrade of the system to allow current and more secure network configuration protocols.
The lack of vendor support is not discussed because the standard OSI and TCP network model are supported by most network device vendors.