## Core Concept
The rule of Hasse is a method used in forensic medicine to estimate the age of a fetus. It is based on the measurement of the length of the fetus. This method is crucial in determining gestational age, which has significant implications in various clinical and legal contexts.

## Why the Correct Answer is Right
The rule of Hasse states that the length of the fetus in centimeters roughly corresponds to the gestational age in weeks after the 12th week of gestation. For example, a fetus measuring 16 cm in length would be approximately 16 weeks old. This method, while not extremely precise, provides a useful estimate. The correct answer, therefore, relates to the estimation of fetal age.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because, although fetal development and size are related to gestational age, the rule of Hasse specifically pertains to estimating age from size, not directly assessing viability or other factors.
- **Option B:** This option is incorrect because, while the sex of the fetus can be determined through ultrasound or genetic testing, the rule of Hasse is not used for this purpose.
- **Option C:** This option might seem plausible because it is related to fetal development, but it is not directly related to what the rule of Hasse determines.
- **Option D:** This option is incorrect because the determination of the cause of death is a complex process involving autopsy and other investigations, not the measurement of fetal size.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the rule of Hasse is specifically used for estimating fetal age based on its length, particularly useful in forensic investigations or when the gestational age needs to be estimated quickly.

**Correct Answer: C. fetal age**