Final answer:
Option B), Pair-rule genes (B) are responsible for defining all segments of the fruit fly's body and dividing them into anterior and posterior halves during embryonic development.
Step-by-step explanation:
The class of segmentation genes in Drosophila that defines all segments of the body and divides each segment into anterior and posterior halves is pair-rule genes (B). This class of genes is critical in the early embryonic development of the fruit fly, operating after the gap genes to refine the segmented pattern of the embryo.
Segmentation genes in Drosophila include gap genes, pair-rule genes, segment polarity genes, and homeotic genes, each playing a specific role in the development of the organism. While it is the pair-rule genes that are responsible for establishing the alternating segment pattern, the segment polarity genes further subdivide each segment and the homeotic genes determine the identity of each segment.