Final answer:
By eight weeks of development, an embryo has all main internal organs formed, as well as major external body structures. Organ development starts around the fourth week, with significant changes visible by the eighth week, where the term fetus is then applicable. The correct option is d. Eight weeks
Step-by-step explanation:
By the time the embryo is eight weeks old, all the main internal organs have formed, along with major external body structures. The developmental process of the embryo is characterized by significant growth and organ formation within the first trimester. During the 4th week after fertilization, the body's organs begin to form, and by the 8th week, all the major organs have started to develop.
The appearance of the embryo also changes during this time; the face begins to look human, external ears start to form, and by week eight, the embryo also starts to show signs of movement. Exposure to toxins at this stage is particularly dangerous due to the critical development of body organs and structures.
By the end of the eighth week, the term fetus becomes applicable as the body is essentially formed, and though organs such as the lungs and liver are not yet functioning, their basic structure is in place. The correct option is d. Eight weeks