About Indicating, brake lights and hazard lights: which statement is correct?

You always indicate about 30 metres, or roughly 3 seconds, before the turn — that is THE rule of thumb.
Once you know the right distance for indicating, that one distance applies on every road — motorway, provincial road or village street, it makes no difference.
✓ Only to avert imminent danger may you give an acoustic or light signal.
Inside the built-up area you indicate only at the very last moment, just before you steer in.

Correct answer: Only to avert imminent danger may you give an acoustic or light signal.

Explanation

Only to avert imminent danger may you give an acoustic or light signal. Alerting others to your presence without danger is not covered by that

There is no '30 metres' and no 'seconds' in the book. The real rule of thumb has three branches by road type: about 300 metres on motorways, about 200 metres outside built-up areas, about 100 metres inside built-up areas
The rule of thumb is explicitly three-part, tied to road type: 300 metres (motorway), 200 metres (outside built-up area), 100 metres (inside built-up area). One distance for all roads is not what the book teaches
That gives other road users no time to react; the rule of thumb is about 100 metres in advance, not at the last moment