Heterozygous tall plant is selfed. It produces both tall and dwarf plants. This confirms Mendel's:

The correct option is B Law of segregation
According to the law of segregation, alleles of a trait that remain together in an individual do not get mixed, but rather keep their identity intact. They separate during gamete formation and get randomly distributed only to get paired again in the offspring’s as per the principle of probability. This results in a phenotypic trait in the $3:1$ ratio in the $F_2$ generation that is, when a heterozygous plant is crossed. So, the correct answer is 'Law of segregation'