The correct answer is A because using preemptible instances within a managed instance group allows for the addition of GPUs at a reduced cost. Preemptible VMs offer significant savings but may be terminated if Compute Engine needs the resources. They're ideal for fault-tolerant applications like batch processing or, in this case, 3D rendering tasks that can handle interruptions. GPUs attached to preemptible instances are also available at lower prices, making this option both cost-effective and suitable for the task. Options C and D are incorrect as App Engine does not currently support GPUs. Option B is less ideal because unmanaged instance groups lack autoscaling capabilities, which are crucial for handling varying workloads efficiently.