What configuration is essential for ensuring the security of subflows?

Setting appropriate access roles for the 'Runs As' user is crucial for securing subflows because this configuration directly controls who can execute the flow and access its underlying data. By assigning specific access roles to the 'Runs As' user, organizations can limit the visibility and manipulation of sensitive data, ensuring that only authorized personnel can execute the subflows and access their results. This is particularly important in environments where sensitive or critical data is being processed, as it helps mitigate risks associated with unauthorized access.

The other options, while related to security, do not directly address the specific needs of subflow execution within ServiceNow Flow Designer. For instance, enabling SSL encryption enhances the security of data transmission across networks but does not regulate who is permitted to run the subflow. Two-factor authentication adds a layer of security for user access but is not specifically tied to controlling flow execution permissions. Limiting flow execution to a single user session could restrict usability and is not a necessary configuration for securing subflows. Hence, assigning access roles is the most effective and essential strategy for safeguarding subflows.