The next step to enable the communication between the forwarder and the deployment server after installing a new forwarder with a manually created deploymentclient.conf is to restart Splunk on the deployment client.
The deploymentclient.conf file contains the settings for the deployment client, which is a Splunk instance that receives updates from the deployment server. The file must include the targetUri attribute, which specifies the hostname and management port of the deployment server. To apply the changes in the deploymentclient.conf file, Splunk must be restarted on the deployment client. Therefore, option C is the correct answer.
References: Splunk Enterprise Certified Admin | Splunk, [Configure deployment clients - Splunk Documentation]