Why can a tall-appearing pea plant not always be considered pure tall?

Correct answer: Tallness can appear when the plant has one dominant and one recessive allele.

The governing distinction is between phenotype and genotype. If T is the dominant tall allele and t is the recessive dwarf allele, both TT and Tt plants appear tall. Only TT is pure or homozygous, whereas Tt is hybrid or heterozygous and can pass the recessive t allele to offspring. Since appearance alone cannot distinguish TT from Tt, a tall plant is not necessarily pure. Therefore option A is correct.