What happens while Winston and Julia are hidden "safely" away in their room watching the red-armed woman sing outside their window?
They realize they are doomed and little hope exists to ever change their
situation.
They realize there is hope for them to overthrow the Party and survive amongst
the proles.
They realize a telescreen is hidden behind the picture of St. Clement's Church,
and that Mr. Charrington is a member of the Thought Police.
They realize a telescreen is hidden in their room, and that O'Brien is a member of
the Thought Police.

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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