**Core Concept**
The question is referring to the sex ratio at birth, which is a demographic measure that indicates the number of males born per 100 females born in a given population. This ratio is an important indicator of population health and fertility patterns.

**Why the Correct Answer is Right**
The correct male to female sex ratio at birth is approximately 105 boys per 100 girls. This ratio is influenced by various factors, including genetics, prenatal care, and environmental exposures. The slight excess of males at birth is thought to be due to the presence of the Y chromosome, which carries the sex-determining region (SRY) gene responsible for male development. The excess of males is also influenced by the X-inactivation process, where one of the two X chromosomes in females is randomly inactivated to avoid a doubling of gene expression.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided in the question, as only the correct answer is given.
**Option B:** Incorrect because the sex ratio at birth is not 90 boys per 100 girls. This ratio would indicate a significant imbalance in male births.
**Option C:** Incorrect because the sex ratio at birth is not 120 boys per 100 girls. This ratio would also indicate an imbalance in male births, which is not typically observed.

**Clinical Pearl / High-Yield Fact**
A useful mnemonic to remember the approximate sex ratio at birth is "105 boys for every 100 girls, a slight excess in males, it's a fact to recall".

**Correct Answer: D. 105 boys per 100 girls.**