How are load balancers used in modern application deployments?

Turning off traffic and taking down compute units for updates represents a downtime-prone deployment strategy, which load balancers are specifically designed to avoid.

Load balancers are crucial for enabling blue/green deployments or rolling updates. They continuously monitor the health of backend compute units and can route traffic away from failing or updating instances, directing it to new or healthy ones, ensuring continuous application availability without downtime during scaling or updates.

This option describes a scenario where old units are discontinued before new ones are fully available, which would cause downtime or service degradation, precisely what load balancers prevent.

While load balancers facilitate bringing up new units and switching traffic, option B better encompasses the continuous nature ("allow traffic to continue") and the full lifecycle of updating and replacing old units, which is the primary benefit of using a load balancer in such a scenario.