If a dwarf plant is treated with gibberellin, and it becomes tall and this plant is now crossed with pure tall plant(TT), then the progeny of the first generation (F₁) is

If a dwarf plant (tt) is treated with gibberellin and becomes tall, and it is crossed with a pure tall plant (TT), then the progeny of the first generation (F₁) will all be tall (Tt).

The original dwarf plant is homozygous for the recessive allele for height, represented as "tt," which results in the dwarf phenotype.
When treated with gibberellin, the dwarf plant's growth is induced, and it becomes tall, but the genotype remains "tt."
The pure tall plant used in the cross is homozygous for the dominant allele for height, represented as "TT."
When the tall plant (TT) is crossed with the treated tall plant (tt), all the offspring (F₁ generation) will receive one "T" allele and one allele of 't' from each parent, resulting in the genotype "Tt."
The F₁ generation individuals will be heterozygous for the height gene, with one dominant "T" allele and one recessive "t" allele. However, phenotypically, they will all be tall because the dominant "T" allele masks the expression of the recessive "t" allele.