**Core Concept:** Heavy placenta, also known as placenta praevia, is a medical condition characterized by the low-lying placenta that partially or completely covers the cervix. This can lead to life-threatening complications during childbirth, such as heavy bleeding due to uterine artery occlusion.

**Why the Correct Answer is Right:** Placenta praevia is primarily caused by placental implantation in an abnormal position during pregnancy. In this case, the correct answer is **D** - Saline Solution (hypertonic saline). Hypertonic saline dilutes blood and causes vasoconstriction, thereby reducing blood flow to the uterus and preventing the uterus from contracting. This helps in preserving the uterine involution and preventing excessive bleeding.

**Why Each Wrong Option is Incorrect:**

A. **Lithium Carbonate (Li2CO3)** is an anti-psychotic medication used to treat bipolar disorder. It does not contribute to heavy placenta or cause abnormal placental implantation.

B. **Calcitonin** is a hormone involved in regulating blood calcium levels. It does not affect placental implantation or cause heavy placenta.

C. **Sodium Chloride (NaCl)** is a salt and does not influence placental implantation or lead to heavy placenta.

**Clinical Pearl:** Heavy placenta can be a life-threatening obstetric emergency. Regular prenatal checkups and proper counselling can help identify high-risk pregnancies and prevent severe complications.