Option A: Tt and Tt
When two tall plants with genotype Tt are crossed, offspring are TT : Tt : tt in ratio 1:2:1, producing 25% dwarf (tt) plants along with tall plants.
Key logic: dwarf plants appear in offspring only if both parents carry the recessive allele 't'. A Tt × Tt cross gives ¼ tt (dwarf). Option B (Tt × tt) gives 50% dwarf — too many. Options C and D cannot produce dwarf offspring at all (TT is present). Examiners expect you to recall the Tt × Tt cross from Mendel's F1 self-pollination experiment.