Diego suddenly lets go of the rope while Anya keeps pulling just as hard. What will most likely happen to Anya?

Anya pulls the rope towards herself (let's say to the left). The rope pulls Anya to the right. Diego pulls the rope towards himself (to the right). The rope pulls Diego to the left. Since they are pulling with the same force and the rope is not moving, the system is in equilibrium. Anya is braced against the pull of the rope.

Diego suddenly lets go of the rope. This means the force Diego was exerting on the rope (and thus the tension in the rope from Diego's side) is removed.

Anya keeps pulling just as hard. Her body was braced to counteract the tension in the rope caused by both her pull and Diego's opposing pull. When Diego lets go, the opposing force on the rope vanishes. Anya's continued pull on the rope, which is no longer balanced by Diego's pull, will cause the rope to accelerate rapidly towards her. Because she is still actively pulling and her body was prepared for resistance, she will lose her balance and stumble forward in the direction she was pulling the rope. This direction is towards where Diego was standing.

• A. Anya stumbles forward, toward where Diego was standing. This matches our analysis, as her pulling force is no longer opposed.
• B. Anya stumbles backward, away from where Diego was standing. This would happen if she suddenly stopped pulling or if the rope broke and she lost her balance backward.
• C. Anya stays in place because no forces act on her anymore. This is incorrect; she is still pulling the rope.
• D. Anya stays in place because the rope still balances her pull. This is incorrect; the balance is broken because Diego is no longer pulling.

The most likely outcome is that Anya will stumble forward.

The final answer is $\boxed{\text{A}}$