The correct answer is option 'A': Heterosis.

Heterosis, also known as hybrid vigor, refers to the phenomenon where the offspring of two different parents show superior traits compared to either parent. It is commonly observed in hybrid plants and animals. This superiority can manifest in various traits, such as increased growth rate, improved resistance to diseases, enhanced fertility, and higher yield.

Heterosis occurs due to the combination of different alleles from each parent, resulting in a more favorable gene pool in the hybrid offspring. Let's understand this concept in detail:

1. Definition of Heterosis:
- Heterosis refers to the increased performance or superiority of hybrids compared to their parents in one or more traits.
- It is a result of the combination of favorable alleles from each parent, leading to improved genetic diversity and fitness in the hybrid offspring.

2. Genetic Basis of Heterosis:
- In a population, different individuals may have different alleles for a particular trait.
- Some alleles can be dominant, conferring advantageous traits, while others may be recessive or less favorable.
- When two different parents with complementary dominant alleles are crossed, the hybrid offspring inherits the best alleles from both parents, resulting in superior traits.

3. Mechanisms of Heterosis:
- There are several proposed mechanisms for heterosis, including:
a) Overdominance: The hybrid inherits two different dominant alleles, resulting in a higher level of gene expression and enhanced trait performance.
b) Epistasis: The interaction between different genes in the hybrid leads to the expression of novel traits that are not observed in either parent.
c) Complementary gene action: Different genes from each parent work together to produce a better outcome in the hybrid.
d) Superiority of hybrids in specific environments: Hybrids may possess traits that are advantageous in particular environments, leading to improved performance.

4. Significance of Heterosis:
- Heterosis plays a crucial role in agriculture and animal breeding.
- Hybrid crops often exhibit higher yields, improved resistance to diseases, and better adaptability to environmental conditions.
- Hybrid animals may have enhanced growth rates, fertility, and resistance to various diseases.
- Heterosis is utilized in breeding programs to develop improved varieties and breeds with desirable traits.

In conclusion, heterosis is the phenomenon where hybrid offspring exhibit superior traits compared to either parent. This occurs due to the combination of favorable alleles from each parent, resulting in improved genetic diversity and fitness in the hybrid. Heterosis plays a significant role in agricultural and breeding programs, contributing to improved productivity and sustainability.