The answer is tall; Tt or TT.
Alleles:
T - dominant allele for tall plants
t - recessive allele for short plants
Genotypes
TT - tall plants
Tt - tall plants (in dominant trait, only one dominant allele is necessary for expression of dominant trait).
tt - short plants
Genotype is the entire set of genes in the organism. Phenotype is the physical characteristics of an organism which are the result of interaction between genotype and environment.
Since pea plants are tall, their phenotypes are tall. Genotypes can be either TT or Tt