The correct option is B Anus
The opening of gastrula by which an organism communicates with the external environment is called as blastopore. During maturation of some animals, it evolves into either anus or the mouth.
In protostome, the blastopore becomes the mouth.
In deuterostome, the blastopore becomes the anus.
Frog is the deuterostome and hence, the blastopore becomes the anus.
Thus, the correct answer is option C.