How do I restrict access to a given command for instance /opt/apps/start, to authorized users only under Linux / UNIX / BSD operating system?

You need to use traditional Unix groups concept to enhance security including restricted access to a given command.

Step # 1: Create and Maintain a Group For All Authorized Users

Create a group named appsonly:
# groupadd appsonly

Add all authorized users to appsonly:
# usermod -aG {groupName} {userName}
# usermod -aG appsonly tom
# usermod -aG appsonly jerry
# id jerry

Where,

1. -a : Add the user to the supplemental group(s) i.e. appends the user to the current supplementary group list.
2. -G : A list of supplementary groups which the user is also a member of.

Step #2: Restrict Access

Now a group of user had been created. Next, use the chgrp command to change the group of /opt/apps/start to appsonly group:
# chgrp {groupName} {/path/to/command}
# chgrp appsonly /opt/apps/start

Disable the file permission for others

Finally, use the chmod command to change file permission as follows:
# chmod 750 /path/to/command
# chmod 750 /opt/apps/start

You can also apply permissions to directory (this will disable ls command access to others) :
# chgrp appsonly /opt/apps
# chmod 0640 /opt/apps

Step # 3: Test It

su to tom, enter:
# su - tom
$ id
$ /opt/apps/start
$ exit

su to vivek (not a member of appsonly group), enter:
# su - vivek
$ id
$ /opt/apps/start

Sample outputs:

bash: /opt/apps/start: Permission denied

A Note About ACL and SELinux

The access control policies which can be enforced by chmod, chgrp, and usermod commands are limited, and configuring SELinux and fille system ACLs (access control list) is a better and recommend option for large deployments.