Answer:
A snake coiled and ready to strike
Step-by-step explanation:
When the author wants to use a symbol to increase tension, this author must write a situation that makes the reader apprehensive and curious. Following this reasoning, the only phrase that leaves the reader apprehensive is "A snake coiled and ready to attack."
You see, a snake's attack is something that creates apprehension, fear, and tension. For that reason, this would be a good symbol for the author to increase the tension at the end of the scene.