The correct option is D. Heterozygous tall. The question refers to a classic Mendelian monohybrid cross where a homozygous tall plant (TT) is crossed with a homozygous short plant (tt).

When a homozygous tall parent (TT) is crossed with a homozygous short parent (tt), all offspring in the F1 generation receive one 'T' allele and one 't' allele, making their genotype all heterozygous (Tt). Because the tall allele (T) is dominant over the short allele (t), all the F1 offspring will express the tall phenotype (observable trait).