According to Darwin's theory of evolution, more individuals are produced in each generation that can survive. Phenotypic variation exists among individuals and the variation is heritable. Only the organisms best adapted to their environment tend to survive and transmit their genetic characters in increasing numbers to succeeding generations while those less adapted tend to be eliminated.

The theory of natural selection, proposed by Charles Darwin, is one of the fundamental principles of evolutionary biology. It explains how species evolve and adapt to their environment over time. The correct answer to the question is option B, which states that natural selection acts on favorable variations.

Natural selection is the process by which certain traits or characteristics become more or less common in a population over successive generations. It operates on the principles of variation, heredity, and differential reproductive success. Here is a detailed explanation of why option B is the correct answer:

1. Variation:
- Natural selection requires genetic variation within a population. Individuals within a species exhibit differences in their traits, such as color, size, or behavior. These variations can arise through random genetic mutations, genetic recombination during sexual reproduction, or other mechanisms.

2. Environmental Pressures:
- The environment exerts selective pressures on individuals with different traits. These pressures can include factors such as predation, competition for resources, disease, or changes in climate. Some traits may provide advantages in certain environmental conditions, while others may be detrimental.

3. Differential Reproductive Success:
- Individuals with advantageous traits are more likely to survive and reproduce, passing on their favorable traits to the next generation. This is known as differential reproductive success. The individuals with less advantageous traits are less likely to survive and reproduce.

4. Favorable Variations:
- In every generation, there are individuals with variations that are better suited to the environment and provide a reproductive advantage. These variations may be physical adaptations, such as longer limbs for better running speed, or behavioral adaptations, such as mating displays or hunting strategies. Over time, these favorable variations become more common in the population.

5. Accumulation of Changes:
- As favorable traits become more prevalent, the overall genetic makeup of the population changes. This is because the individuals with advantageous traits are passing on their genes to the next generation, while those with less advantageous traits contribute fewer genes to the gene pool.

6. Gradual Change:
- Natural selection is a gradual process that occurs over many generations. It does not lead to overnight changes or the sudden appearance of new species. Instead, it operates slowly and incrementally, resulting in the accumulation of changes within a population.

In conclusion, the theory of natural selection focuses on the idea that natural selection acts on favorable variations within a population. This process gradually changes the gene frequency from one generation to another, leading to the adaptation and evolution of species over time.