Final answer:
The nutrient deficiency that can lead to decreased birthweight because of low bone mineral density in a fetus is calcium. Therefore, the correct option is c.
Step-by-step explanation:
The nutrient deficiency in the pregnant adolescent that may result in decreased birthweight due to low bone mineral density in the fetus is calcium. A fetus requires calcium for proper bone development, and insufficiency can lead to lower birthweight. Additionally, vitamin D is essential for calcium absorption, and its deficiency can lead to rickets, a condition marked by improper bone development. While folic acid is crucial for preventing neurological defects like spina bifida, it's specifically calcium that's instrumental for bone mineral density.