I would want one of my partition to be mounted read only always... How to do that ?

1. /etc/fstab

I know that, in options we can give ro -- so when the machine boots up or 'mount -a' it will be mounted read only. But i dont want that to be mounted when machine boots up.

We will mount that when only required !

2. announcing all users

I can announce all users to use ro mount always, but that is not practical as they may not do it.

As there is sensible data, which for sure we will use only for reading..

Finally, How to force read only mount always for a specific partition ?

You may want to export the partitions in question as NFS read-only (?).

I can take this one of the suggestion, but it would really be good to get some other good solution.

Bcoz, basically NFS is meant for some other purpose.

Hello, seeing that mount can only be done as root .
you can simply remove that entry from fstab.