Explanation:
The principle of segregation, also known as Mendel's first law, states that during gamete formation, the alleles for a trait segregate or separate from each other so that each gamete carries only one allele for each trait. This principle explains how offspring inherit traits from their parents and provides the basis for understanding genetic inheritance patterns.

In the given scenario, the plant is heterozygous for tallness, which means it has one allele for tallness (T) and one allele for dwarfness (t). When this plant undergoes gamete formation, the two alleles segregate from each other and move into separate gametes. This results in half of the gametes containing the T allele and the other half containing the t allele.

F1 Generation:
When the heterozygous plant self-fertilizes, the gametes carrying the T allele can combine with gametes carrying the T or t allele, and the gametes carrying the t allele can also combine with gametes carrying the T or t allele. As a result, four possible combinations can occur in the F1 generation: TT, Tt, tT, and tt.

Principle of Segregation in F2 Generation:
When the F1 generation plants are crossed, the possible combinations of alleles can be observed in the F2 generation. In this case, the F1 plants can produce gametes with either the T or t allele. When these gametes combine during fertilization, the following genotypes can occur in the F2 generation:

- TT (tall)
- Tt (tall)
- tT (tall)
- tt (dwarf)

The appearance of both tall and dwarf plants in the F2 generation confirms the principle of segregation. The alleles for tallness (T) and dwarfness (t) segregated during gamete formation in the F1 generation, allowing for the possibility of both alleles being expressed in the F2 generation.

Therefore, the correct answer is option B, segregation.