The best argument to support what will happen to the object placed between the positively and negatively charged metal plates is based on the concept of the electric field created by the charged plates.
When the positively charged plate and the negatively charged plate are positioned close to each other, an electric field is established between them. This electric field exerts a force on any charged object placed within it. If the object is neutral, it may experience an induced charge distribution, where positive and negative charges within the object redistributing themselves due to the external field. However, if the object is charged (either positively or negatively), it will experience a force that can be determined using Coulomb's law.
If the object is positively charged, it will be attracted towards the negatively charged plate. Conversely, if it is negatively charged, it will be attracted towards the positively charged plate. Thus, depending on the charge of the object, it will move towards the plate with the opposite charge due to the force exerted by the electric field established between the plates.
Overall, the argument that the electric field between the two plates exerts a force on charged particles (or induces charges on neutral particles) best supports what will happen to the object placed in this region.