The valid alternative to maintain a deployed proxy technology while saving physical space in the data center by moving the network service to the virtualization infrastructure is NFV (Network Function Virtualization).
NFV is a technique that allows network functions, such as proxies, firewalls, routers, or load balancers, to be implemented as software applications running on virtual machines or containers. NFV reduces the need for dedicated hardware devices and improves scalability and flexibility of network services. References: CompTIA Network+ N10-008 Certification Study Guide, page 440; The Official CompTIA Network+ Student Guide (Exam N10-008), page 16-11.
NFV can be used to virtualize a wide variety of network functions, including proxy servers. By virtualizing proxy servers, organizations can save physical space in the data center and improve the scalability and efficiency of their networks.
To virtualize a proxy server using NFV, an organization would need to deploy a virtualization platform, such as VMware ESXi or Microsoft Hyper-V. The organization would then need to install a virtual proxy server appliance on the virtualization platform.
Once the virtual proxy server appliance is installed, it can be configured and used just like a physical proxy server.
NFV is a relatively new technology, but it is quickly gaining popularity as organizations look for ways to improve the efficiency and scalability of their networks.

Explanation
A clientless VPN is a method of providing remote clients with access to an internal web application without installing any additional software or dedicated VPN client on their devices. Instead, users access the VPN through a web browser, utilizing a web portal or gateway provided by the VPN service. This method provides the highest flexibility and compatibility, as it supports various operating systems and devices, and encrypts only the connection to the web application, not the entire traffic of the device.

A NIC is a network interface card that enables a device to connect to a network using a wired or wireless medium. A NIC has a specific speed and bandwidth capacity that determines how much data it can transmit and receive per second. A higher-throughput NIC can handle more traffic than a lower-throughput NIC.
LACP is a link aggregation protocol that allows multiple physical links to be combined into a single logical link. LACP can increase the bandwidth, reliability, and load balancing of a network connection by distributing traffic across multiple links. LACP is also known as IEEE 802.3ad or port channeling.
A network administrator can install an additional NIC on a server and configure LACP to combine the two NICs into one logical link. This can effectively double the throughput of the server's network connection and allow it to handle more traffic without exceeding 1GB. This is the best solution for this issue because it improves the performance and availability of the server without affecting its functionality or requiring additional hardware.
The other options are not the best solutions for this issue because they either do not increase the throughput of the server's network connection, or they have negative impacts on the server's functionality or performance. Removing some of the applications from the server may reduce the traffic generated, but it may also affect the server's role and service delivery. Configuring the NIC to use full duplex may improve the efficiency of the network communication, but it does not increase the bandwidth capacity of the NIC. Configuring port mirroring to send traffic to another server may help with monitoring or troubleshooting, but it does not reduce the traffic load on the original server. Installing a SSD may decrease the data processing time of the server, but it does not affect the network throughput of the server. Reference:
CompTIA Network+ N10-008 Study Guide, Chapter 2: Network Devices and Technologies, Section 2.2: Network Device Functions and Features, Subsection: Network Interface Cards, pp. 72-74.
CompTIA Network+ N10-008 Study Guide, Chapter 2: Network Devices and Technologies, Section 2.3: Network Cabling and Connectors, Subsection: Link Aggregation, pp. 87-88.
Professor Messer's CompTIA N10-008 Network+ Course Notes, Section 2.2: Network Devices, Subsection: Network Interface Cards, p. 28.
Professor Messer's CompTIA N10-008 Network+ Course Notes, Section 2.3: Network Cabling and Connectors, Subsection: Link Aggregation, p. 31.