Final answer:
The Osiris kids cannot leave Serenity because there is a security lockdown in place.
Step-by-step explanation:
The Osiris kids cannot leave Serenity because there is a security lockdown in place.
In the passage referenced, it mentions that the master-at-arms has locked the prisoners in the after-hold, stopping them from reaching the magazine.
This indicates that there is restricted access and security measures in Serenity that prevent the Osiris kids from leaving.
Therefore, option B) There's a security lockdown is the correct answer.