Final answer:
In the Platonic theory of forms, God (a) is the correct answer as to what exists in the world of 'form'. This invisible realm is where the perfect and eternal forms or essences exist, and God, being the ultimate perfect being, is considered to exist in this realm of reality.
Step-by-step explanation:
The question pertains to the notion of what exists in the world of 'form' according to philosophical theories about the nature of reality. Specifically, it refers to the Platonic theory of forms, a central aspect of the philosophy of Plato who believed in a dualistic existence of the visible material world and an invisible, eternal realm of forms or ideas. This invisible realm holds the perfect and eternal 'form' of all things that exist in our changing material world.
In the context of the theory of forms, reality is what exists eternally as immaterial forms or essences that are unchanging and perfect. These forms are the true essence of what we perceive in the physical world. Based on this concept, God, as an ultimate and perfect being, would be said to exist in the realm of forms or reality. As argued by philosophers such as Anselm of Canterbury (in his ontological argument), if God is a being than which nothing greater can be conceived, and if existing in both the mind and in reality is greater than existing solely in the mind, then God must exist in reality – in the world of forms.
Therefore, in the context of Platonic philosophy, God (a) can be said to be in existence in the world of 'form'. Plato’s dialogues suggest that the world of forms contains the perfect, immutable essences of things, which are more real than their physical counterparts.