Final answer:
Vitamin D supplements are given to prevent rickets in preterm newborns, as a deficiency leads to improper bone development.
Step-by-step explanation:
In order to prevent rickets in the preterm newborn, the supplement that is given is Vitamin D. Rickets is a childhood disorder characterized by improper bone development, and it is caused mainly due to a deficiency of Vitamin D, which is crucial for stimulating the intestinal absorption of calcium and phosphate. This vitamin is essential for healthy bone formation in newborns, and its deficiency can lead to bowlegs and other bone deformities due to defective mineralization. It is important to note that while other supplements such as Vitamin C, folic acid, and iron are important for overall health, they do not prevent rickets specifically.