What is Twilly's signal to Mrs. Starch that it is all clear?

Twilly’s signal to Mrs. Starch that it is all clear is indicated by no response to his whistle. This means that if there is silence following his signal, it signifies that the situation is safe and that Mrs. Starch can proceed without concern.

This method of communication relies on the absence of sound, which serves as a clear indication in circumstances where visual signals may not be practical or reliable. The choice of a non-verbal signal helps in maintaining stealth and discretion, which is essential in the context Twilly and Mrs. Starch find themselves in.

The other options involve direct auditory signals such as a shout, claps, or a whistle that would contradict the need for subtlety in their communication. Utilizing silence as a signal emphasizes the need for caution and awareness of their surroundings.