The correct answer is: The third charge will be in unstable equilibrium.
If a third equal and similar charge is placed between two equal and similar charges, then the third charge will experience a force from each of the two charges. The force from each charge will be in the opposite direction, and the magnitude of the force will be the same. This means that the third charge will be in equilibrium, but it will be an unstable equilibrium. This means that if the third charge is displaced from its equilibrium position, it will move away from the equilibrium position.
Option A is incorrect because the third charge will not move out of the field of influence of the two charges. The third charge will be in the field of influence of the two charges, and it will experience a force from each of the two charges.
Option B is incorrect because the third charge will not remain in stable equilibrium. The third charge will be in equilibrium, but it will be an unstable equilibrium. This means that if the third charge is displaced from its equilibrium position, it will move away from the equilibrium position.
Option C is incorrect because the third charge will be in equilibrium. The third charge will experience a force from each of the two charges, but the forces will be equal in magnitude and opposite in direction. This means that the third charge will be in equilibrium.
Option D is correct because the third charge will be in unstable equilibrium. The third charge will experience a force from each of the two charges, but the forces will be equal in magnitude and opposite in direction. This means that the third charge will be in equilibrium, but it will be an unstable equilibrium. This means that if the third charge is displaced from its equilibrium position, it will move away from the equilibrium position.