**Core Concept**
The basi-occipital bone fuses with the basi-sphenoid bone in the human skull, a process that occurs during the growth and development of an individual. This fusion is an essential aspect of skeletal maturation, which is crucial in forensic medicine for estimating age based on skeletal remains.

**Why the Correct Answer is Right**
The fusion of the basi-occipital bone with the basi-sphenoid bone typically occurs at the age of 20 years in females. This process is part of the normal ossification pattern of the human skull, which involves the gradual union of separate bone fragments into a single, solid bone. The basi-occipital bone and basi-sphenoid bone fuse to form the basisphenoid bone, a critical component of the occipital bone. This fusion is an important milestone in skeletal development, and its timing can be used as an indicator of age in forensic anthropology.

**Why Each Wrong Option is Incorrect**
**Option B:** 23 years - This option is incorrect because the fusion of the basi-occipital bone with the basi-sphenoid bone typically occurs before the age of 23 years in females.
**Option C:** 27 years - This option is also incorrect because the fusion of the basi-occipital bone with the basi-sphenoid bone typically occurs before the age of 27 years in females.
**Option D:** 17 years - This option is incorrect because the fusion of the basi-occipital bone with the basi-sphenoid bone typically occurs after the age of 17 years in females.

**Clinical Pearl / High-Yield Fact**
In forensic medicine, the timing of skeletal fusions, such as the fusion of the basi-occipital bone with the basi-sphenoid bone, can be used to estimate the age of an individual based on skeletal remains. This knowledge is essential for forensic anthropologists and medical examiners who need to determine the age of a deceased person in cases of violent death or natural disasters.

**✓ Correct Answer: A. 20 years**