Which of the following systems first begins to take shape in a frog?

The answer is A, as with all higher animals. When the embryo is fertilized and forms the zygote, it divides to form a hollow ball of cells which then undergoes gastrulation, where there is an infolding of cells, and a pore is formed: this will be the the start of the digestive system. On the out surface a furrow is formed, this is the neural tube, the beginning of the nervous system.