Final answer:
Margot's feeling that the world has become very strange before a panic attack is termed derealization, a sensation of the external world seeming unreal or distorted.
Step-by-step explanation:
Derealization disorder involves a persistent or recurring feeling of being detached from one's body or mental processes, like an outside observer of one's life (depersonalization), and/or a feeling of being detached from one's surroundings (derealization).
Just before she experiences a panic attack, Margot feels that the world has suddenly become very strange. This feeling is termed derealization, which is option a). Derealization is a sensation of unreality concerning the external world, making it seem distorted or surreal.
It is often associated with panic attacks and other types of anxiety disorders. Depersonalization, on the other hand, relates to a sense of detachment from oneself, which was not the effect described by Margot. Phobia and obsession are unrelated to the sensation of the world feeling strange.