Why is there no 'Recovery HD' listed when holding 'option' on startup, however, 'Recovery HD' is shown in terminal with 'diskutil list' command? When I hold 'option' during boot I only am shown a 'Macintosh HD' disk to boot from. There is no 'Recovery HD' available. 

Using the 'diskutil list' command in terminal reveals that I do have a Recovery HD partition.

/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *251.0 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:          Apple_CoreStorage Macintosh HD            250.1 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
/dev/disk1 (internal, virtual):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                  Apple_HFS Macintosh HD           +249.8 GB   disk1
                                 Logical Volume on disk0s2
                                 36DB2BF6-94A5-4FAA-8D39-94E622E2597F
                                 Unencrypted


Why is the 'Recovery HD' not displayed when I want to choose my startup disk by holding 'option' on boot?

MacBook Pro Retina running OS EL Capitan 10.11.6

Note I found this idea while searching, that suggests converting my Apple_CoreStorage Macintosh HD with virtual Apple_HFS drive into a physical Apple_HFS drive using the command $ diskutil corestorage revert /dev/disk1. The OP couldn't boot into Recovery mode but was also having problems booting into any HDD with OS X (while I can boot normally into my HDD just fine). https://www.tonymacx86.com/threads/solved-cannot-boot-from-os-recovery-disk.175254/
Holding down the Option Key during boot allows you to choose from "user controlled/defined" startup disks (e.g. your Mac O/S, a Bootcamp O/S, a bootable USB, etc.) whereas holding down Command-R during boot boots from a Recovery Partition that was created during an O/S install. It would be nice if all the boot options were available together, but alas, they are not.