Final answer:
The percentage of reaction from gadolinium contrast injection is not specified in the options; however, an 'Effective dose - 50%' refers to the dose that results in a significant response in 50% of subjects.
Step-by-step explanation:
The percentage of reaction from a gadolinium contrast injection is not provided among the choices given, as it can greatly vary from person to person. However, typical reactions are rare and serious reactions are even rarer.
The question regarding an "Effective dose - 50%" refers to option D, which is the dose that results in a significant response in 50% of subjects. The context of the question seems to imply a potential misunderstanding or mix-up with pharmacological terms such as ED50, which is a different concept from gadolinium contrast reaction rates.