The inference that can be derived when they hid in their room is that D. They realize a telescreen is hidden in their room, and that O'Brien is a member of the Thought Police.
An inference simply means the conclusion that can be deduced based on the evidence given.
In this case, when Winston and Julia are hidden safely away in their room, they realize a telescreen is hidden in their room, and that O'Brien is a member of the Thought Police.
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