Final answer:
Network ACLs are the optional security control that can be applied at the subnet layer of a VPC. They act as a firewall for controlling traffic in and out of subnets and are stateless. Route Tables define paths for traffic but do not serve as a security control.
Step-by-step explanation:
The optional security control that can be applied at the subnet layer of a Virtual Private Cloud (VPC) is Network ACLs (Network Access Control Lists). Network ACLs are an additional layer of security for your VPC that act as a firewall for controlling traffic in and out of one or more subnets. They are stateless, which means that they check the incoming and outgoing packets separately. Unlike Security Groups, which are mandatory and operate at the instance level, Network ACLs are optional and provide a rule-based tool for controlling network traffic.
Route Tables are not a security control but rather define the paths that network traffic can take out of the VPC subnets. While they are essential for network architecture, they do not filter traffic based on security policies like Network ACLs.