We lack info to be sure while picking the right answer, as this is dependent on the initial config of App1. Alternatively we have to assume that App1 has default nodes which are DS2v2 (different from DS3v2 required by App2) I'm guessing the actual exam question has clickable sections and we only see one shot here. Having said that, bear with me:
A - wrong, this is related to Kubernetes software version
B - valid answer, ref. https://docs.microsoft.com/en-us/azure/aks/use-multiple-node-pools
C - probably wrong as App1 and App2 (probably) have different specs
D - virtual nodes are not meant for this use case, but rather to wrap Kubernetes around other services that support containers such as AWS Fargate, IoT Edge, Hyper.sh or Microsoft’s ACI