Which of the following is a method of probing underwater?

Probing underwater is a technique used to locate objects or determine the underwater terrain when visibility is poor or non-existent. Using drag hooks is a valid method of probing because they are specifically designed to snag or hook onto objects submerged in water. This technique involves dragging the hooks across the bottom of the water body, allowing for the identification and retrieval of items that cannot be seen due to murky conditions.

Sonar scanning, while a useful tool for underwater exploration and mapping, primarily relies on sound waves to detect objects rather than physically probing the water's surface. Human chains and verbal communication are valuable in rescue scenarios but do not inherently involve the act of probing the underwater environment for submerged objects. The focus of probing is on the physical interaction with the water and the bottom substrate, which is why drag hooks is the most suitable method among the choices provided.