All of the forwarders would still be phoning home to the old deployment server, because the forwarders are configured to use the old deployment server in $SPLUNK_HOME/etc/system/local. This is the local configuration directory that contains the settings that override the default settings in
$SPLUNK_HOME/etc/system/default. The deploymentclient.conf file in the local directory specifies the targetUri of the deployment server that the forwarder contacts for configuration updates and apps. If the forwarders have the old deployment server's targetUri in the local directory, they will ignore the updated deploymentclient.conf file that was deployed by the old deployment server, because the local settings have higher precedence than the deployed settings. To fix this issue, the forwarders should either remove the deploymentclient.conf file from the local directory, or update it with the new deployment server's targetUri.
Option C is the correct answer. Option A is incorrect because a version mismatch between the forwarders and the new deployment server would not prevent the forwarders from phoning home to the new deployment server, as long as they are compatible versions. Option B is incorrect because the new deployment server is configured and running, and there is no indication that it is not accepting connections from the forwarders.
Option D is incorrect because the pass4SymmKey is the shared secret key that the deployment server and the forwarders use to authenticate each other. It does not affect the forwarders' ability to phone home to the new deployment server, as long as it is the same on both sides12
1: https://docs.splunk.com/Documentation/Splunk/9.1.2/Updating/Configuredeploymentclients 2:
https://docs.splunk.com/Documentation/Splunk/9.1.2/Admin/Wheretofindtheconfigurationfiles