**Chromosome Theory of XY Sex Determination**

The correct answer is option 'A', Wilson and Stevens.

**Explanation:**

The chromosome theory of sex determination, also known as the XY sex determination system, was proposed by Nettie Stevens and Edmund Beecher Wilson in the early 20th century. This theory explains how the sex of an individual is determined by the presence or absence of certain chromosomes.

**Background:**

Before the discovery of the chromosome theory of sex determination, the mechanism behind the determination of sex was not well understood. It was known that males and females possessed different reproductive organs and exhibited different secondary sexual characteristics, but the exact factors influencing these differences were unclear.

**Stevens' Contribution:**

Nettie Stevens, an American biologist, conducted groundbreaking research on the chromosomes of insects such as mealworms and beetles. In 1905, she made a significant discovery that led to the development of the chromosome theory of sex determination. Stevens observed that certain cells in the testes of mealworms contained one large chromosome (X chromosome), while other cells in the ovaries contained two smaller chromosomes (X and Y chromosomes).

**Wilson's Contribution:**

Around the same time, Edmund Beecher Wilson, an American geneticist, was also studying the role of chromosomes in sex determination. He independently arrived at similar conclusions as Stevens. Wilson hypothesized that the presence of the Y chromosome was responsible for determining maleness, while the absence of the Y chromosome resulted in femaleness.

**Chromosome Theory of XY Sex Determination:**

Based on their respective research findings, Stevens and Wilson proposed the chromosome theory of sex determination. According to this theory:

1. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
2. The presence of the Y chromosome in males determines the development of male reproductive organs and secondary sexual characteristics.
3. The absence of the Y chromosome in females allows for the development of female reproductive organs and secondary sexual characteristics.

**Significance:**

The chromosome theory of XY sex determination provided a solid scientific basis for understanding the genetic basis of sex determination in organisms. It revolutionized the field of genetics and laid the foundation for further research on sex chromosomes and the inheritance of sex-linked traits.

In conclusion, the chromosome theory of XY sex determination was proposed by Nettie Stevens and Edmund Beecher Wilson based on their independent research on the role of chromosomes in sex determination. Their findings and hypothesis formed the basis of our current understanding of how the presence or absence of certain chromosomes determines the sex of an individual.