Which option explains why a user-created local filesystem does not automatically mount when the system boots?

The mountd port in /etc/services is relevant to NFS remote mounts over the network and has no bearing on local JFS or JFS2 filesystem auto-mount behavior.

Mount groups in AIX control the order in which filesystem stanzas are processed and are not a prerequisite for a filesystem to mount automatically at boot.

The /etc/filesystems file in AIX contains a stanza for each filesystem, and the 'mount' attribute within that stanza must be set to 'true' or 'automatic' for the filesystem to be mounted during system initialization. When a user creates a local filesystem manually, this attribute may not be added to the stanza automatically, preventing auto-mount at boot.

The portmap daemon is part of the RPC infrastructure used by NFS and other network services and is completely unrelated to mounting local filesystems during system startup.