Final answer:
To prevent new switches from becoming the elected root in STP environments, you should use the root guard command on switch ports where you want the current root bridge to be maintained.
Step-by-step explanation:
To prevent new switches from being elected root in a network, the command you should enter is b) root guard. This command is used in Spanning Tree Protocol (STP) environments to maintain the current root bridge and to prevent other switches from becoming the root in case they have a better bridge priority. Root guard is applied to ports that are designated or root ports on a switch. When root guard is enabled on a port and a BPDU comes in with superior information, this port will be put into a root-inconsistent STP state, effectively blocking it and thus preventing the switch at the other end from becoming the root bridge.