The correct option is A Horstadius
Original gradient theory was given by T. Boveri and was supported by M. C. Child. Child called it as a metabolic axial gradient theory. According to it, the egg has distinct animal and vegetal poles. The cytoplasm of the animal pole divides rapidly but the rate of cleavage in the vegetal pole is slow. It indicates that the rate of metabolism is more in the animal pole than the vegetal pole which is called as a metabolic axial gradient. Horstadius and Runnstrom called it as double gradient theory. They explain egg development on the basis of animal pole gradient and vegetal pole gradient. Hence, the correct option is D.