Atestcross on a tall pea plant produces only tall offspring. what is the genotype of the tall pea plant?

Test cross is when you cross (aka. "mate" in simple language) the particular pea plant (which you would like to find the genotype of) with a pea plant which is homozygous recessive.

Let's say allele T codes for the protein which synthesizes the growth hormone which makes the pea plant tall. Allele T is dominant to allele t, which does not code for extra growth hormones when in homologous recessive form (tt) as allele T does.

Tall pea plants may have any of the 2 genotypes as follows: TT, Tt.

The test cross of TT:
TT × tt will give all Tt offspring, which satisfies the criteria in the question, where all of the offspring via the test cross are tall.

The test cross of Tt:
Tt × tt will give Tt, tt, a phenotypic ratio of 1 Tall : 1 Short.

Thus, the genotype of the original tall pea plant is TT.

Hope this helps! :)