What is the secret sign that Omar and Amal use to meet at the stream?

In the context of the story, Omar and Amal have established a specific method for communicating secretly, which enhances their bond and shows their creativity in navigating their circumstances. The choice of three knocks serves as a distinctive auditory signal that allows them to connect without drawing the attention of others. This method not only ensures privacy for their meetings but also embodies the themes of friendship and trust present in the narrative.

The other options, while plausible in different contexts, do not hold the same significance or relevance in the specific scenario presented in the story. The use of three knocks as a signal is likely to reinforce the narrative's focus on clever communication and problem-solving among the characters.