**Core Concept:**

Population genetics is a branch of genetics that studies the distribution of genetic variation within and among populations. It focuses on understanding the process of evolution, the genetic diversity within species, and how genetic traits are transmitted from one generation to another. Key concepts include allele frequency, Hardy-Weinberg equilibrium, genetic drift, migration, and natural selection.

**Why the Correct Answer is Right:**

The correct answer, D (Mendel's Laws), represents the foundational principles in genetics that were discovered by Gregor Mendel in the 19th century. These laws explain how traits are inherited from parents to offspring:

1. Segregation: Each allele (gene variant) segregates independently of other alleles, meaning that the presence of one allele does not affect the presence of another allele.
2. Independent Assortment: Alleles are assorted independently during cell division, ensuring that the two alleles for a single gene do not stick together and are distributed to the gametes (sperm and egg cells) separately.

**Why Each Wrong Option is Incorrect:**

A (Mutation): While mutations play a role in genetic variation, they are not the sole focus of population genetics. Population genetics also considers factors like allele frequency, genetic drift, and natural selection.

B (Genetic Drift): Genetic drift is a random change in allele frequencies within a population due to chance events. While it contributes to genetic variation, population genetics encompasses more concepts like Mendelian inheritance and natural selection.

C (Mendelian Ratio): Mendelian Ratio (1:1:1:1) explains the inheritance ratios for dominant and recessive traits. However, it is only a specific aspect of Mendelian inheritance, not the entire domain of population genetics.

**Why the Correct Answer is Right:**

D (Mendelian Laws) encompasses the foundational principles of genetics, including segregation and independent assortment, which are essential for understanding genetic variation and inheritance patterns within and among populations. These laws provide a framework for analyzing genetic traits and their distribution, contributing to the broader field of population genetics.

**Why Each Wrong Option is Incorrect:**

A (Mutation): While mutations contribute to genetic variation, they alone do not encompass the full scope of population genetics, which also considers factors like allele frequency, genetic drift, and natural selection.

B (Genetic Drift): Genetic drift is one aspect of population genetics, but it is not the sole focus, as the field also considers allele frequencies, natural selection, and other genetic factors.

C (Mendelian Ratio): The Mendelian Ratio (1:1:1:1) is specific to the inheritance of certain traits and does not encompass the broader concept of population genetics, which encompasses allele frequencies, genetic drift, natural selection, and other factors affecting genetic variation and inheritance patterns.