Answer:
The opening formed during gastrulation becomes the mouth.
Step-by-step explanation:
Protostomes are the majority of animals classified as triblastics (which have three embryonic leaflets), except for the echinoderms and chordates.
The embryonic leaflets originated in the gastrulation of the triblastics (and consequently of the protostomes) are: endoderm, mesoderm and ectoderm.
In the process of gastrulation of the protostomes, the embryo has a cavity called the archenter (which will give rise to the intestine). The archenter communicates with the external environment through an opening, called blastopore.
In protostomes, the blastopore gives rise to the mouth, unlike deuterostomes, where the blastopore gives rise to the anus.
Given the above, it is concluded that the correct alternative is "the opening formed during gastrulation becomes the mouth".