The correct command is gcloud container clusters resize my-gcp-ace-proj-1 --node-pool my-gcp-ace-primary-node-pool --num-nodes 20. This command uses the --num-nodes parameter to specify the target number of nodes in the cluster, which is the recommended way to resize a node pool. The other options are incorrect for the following reasons:

• Using kubectl with container clusters update is not valid as kubectl does not accept container as an operation.
• The gcloud container clusters update command cannot be used to specify the number of nodes; it's used for updating node locations.
• The gcloud container clusters resize command does not support the --new-size parameter; --num-nodes should be used instead.

References:

• kubectl Overview
• gcloud container clusters update
• gcloud container clusters resize