During gastrulation, two openings form in most animals. if the first opening becomes the mouth, the animal is a

protostome

Explanation:

The fate of blastopore determined whether an animal is protostome or deuterostome.

During Gastrulation, digestive tube starts developing from the archenteron which leads to formation of two openings namely – mouth and anus. A balstopore is responsible for forming the fist opening . Now there arises two cases –

a)When the blastopore forms mouth as the first opening, the anus becomes the second opening (formed from the secondary opening). These organisms are referred to as protostome  

b)When the blastopore forms anus as the first opening, the mouth becomes the second opening (formed from the secondary opening). These organisms are referred to as deutrostome.  

the correct answer is b