The question requires improving the network performance of a security appliance running on a single Amazon EC2 instance. The most direct methods to enhance an EC2 instance's network capabilities are by selecting an instance type with better underlying hardware features and by scaling it vertically.

Using an EC2 instance that supports enhanced networking (SR-IOV) provides significantly higher packets per second (PPS), lower inter-instance latency, and less network jitter. Increasing the EC2 instance size directly correlates to higher available network bandwidth. Larger instance types are allocated a greater share of network capacity, which is crucial for a device acting as a network chokepoint for all outbound traffic.

B. Send outbound traffic through a transit gateway.

A transit gateway is a network transit hub for interconnecting VPCs and on-premises networks. It simplifies routing but does not inherently increase the network performance of the EC2 security appliance itself.

D. Place the EC2 instance in a placement group within the VPC.

Placement groups optimize network performance between instances within the group. This does not improve performance for traffic flowing from the instance to an external location like an on-premises data center.

E. Attach multiple elastic network interfaces to the EC2 instance.

The total network bandwidth for an EC2 instance is determined by its type, not the number of network interfaces. Multiple ENIs do not increase the aggregate throughput of the instance.

1. Amazon EC2 Documentation - Enhanced networking on Linux: "Enhanced networking uses single root I/O virtualization (SR-IOV) to provide high-performance networking capabilities on supported instance types. SR-IOV is a method of device virtualization that provides higher I/O performance and lower CPU utilization when compared to traditional virtualized network interfaces." (Source: AWS Documentation, Enhanced networking on Linux, Section: "Enhanced networking").

2. Amazon EC2 Documentation - Amazon EC2 instance types: The instance type specifications clearly show that network bandwidth scales with instance size. For example, for M5 instances, m5.large has "Up to 10 Gbps" while m5.24xlarge has "100 Gbps". This demonstrates the direct relationship between instance size and network performance. (Source: AWS Documentation, Amazon EC2 Instance Types, Section: "General Purpose").

3. Amazon EC2 Documentation - Placement groups: "A cluster placement group is a logical grouping of instances within a single Availability Zone. ... A cluster placement group is recommended for applications that benefit from low network latency, high network throughput, or both, and if the majority of the network traffic is between the instances in the group." (Source: AWS Documentation, Placement groups, Section: "Cluster placement groups").