During which process is exposure to teratogens most devastating for the development of the embryo's organs

The correct answer is: about 3 weeks after fertilization.

Teratogens are agents, biological, chemical or physical, that can affect normal development causing the formation of anomalies. The critical time for the teratogen’s effects is embryonic development. If teratogens act within two weeks after fertilization (implementation phase), probably the embryo will be lost. Three to eight weeks after fertilization is the embryonic period when primary tissue differentiation occurs and under the influence of teratogen major malformation might occur. During the fetal period (after eight weeks) central nervous system is at risk.

they float because they are less dense than the water itself

something denser than water, like a rock, will sink to the bottom