## **Core Concept**
The question pertains to the ultrasonographic (USG) signs of fetal death. Fetal demise can be confirmed through various ultrasound findings that indicate the cessation of fetal cardiac activity and other morphological changes.

## **Why the Correct Answer is Right**
The correct answer involves understanding the specific USG signs associated with fetal death.
- **Spalding sign**: This refers to the overlapping of the bones of the fetal skull due to the reduction in intracranial volume after fetal death.
- **Halo sign** typically refers to a different context, often related to imaging of the liver or other structures, and is not a standard term for fetal death.
- **Absent heart beat** is a definitive sign of fetal death, as it directly indicates the cessation of cardiac activity.
- **Hegar's sign**, on the other hand, is a clinical sign used to assess early pregnancy by identifying the softening of the lower part of the uterus.

## **Why Each Wrong Option is Incorrect**
- **Option A: Halo' sign of head** - This is not a standard term used for fetal death; the correct term related to fetal demise is more accurately described by other options.
- **Option D: Hegar's sign** - This is a sign of early pregnancy, not fetal death. It is used to detect the softening of the uterine isthmus.
- **Option B: Heart beat absent** is indeed a correct sign of fetal death but let's verify if it's listed as an option in combination.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the **Spalding sign** and **absent fetal heartbeat** are critical USG signs of fetal death. Among these, **Spalding sign** is particularly notable as it is a morphological change that occurs post-mortem.

## **Correct Answer:** C.