He wanted to copyright the idea of half-living, half-robot creatures in another galaxy for later sequels but was denied. What element was missing?

The correct answer is B. Originality.

In copyright law, an idea must be expressed in a specific way to be protected, which means it needs to be original and not just a general concept. If the idea of half-living, half-robot creatures was not presented in a unique form or manner, it lacks the originality required for copyright protection. Always remember that copyright does not protect mere ideas but focuses on the expression of those ideas.