Final answer:
The tension of a curl gives the hair direction and mobility. Tension is a force that acts along a flexible connector, such as rope or cable, and can similarly guide a hair curl.
Step-by-step explanation:
The tension of a curl gives the hair direction and mobility. The correct answer to the student's question is D) Tension. Tension refers to a force along the length of a medium; it is a pulling force that acts along a stretched flexible connector, like a rope or cable. In the context of hair styling, applying tension to a curl can influence the hair's direction and its ability to move freely. This concept is also closely related to the physics of how flexible connectors work, as they can only exert pulls parallel to their length, which is akin to the way tension can guide the direction and movement of a hair curl.