During embryo development, there are mainly 3 stages. These are germinal stage, gastrulation and neurulation. Germinal stage is further divided into fertilization, cleavage, bastulation and implantation. Archenteron formation is preceded by neurulation. It occurs during gastrulation stage. During gastrulation, endoderm, mesoderm and ectoderm - 3 germ layers are formed. After this, blastopore develops in one side of the embryo and it deepens to become the archenteron. Archenteron is the first formative stage of the gut.
Thus, the correct answer is 'Neurulation.'