The answer is 3:1.
If we imagine that plant has two alleles for the trait, we can dominant allele represent with P represents and recessive allele with p. To get purebred monohybrid in the first generation, parents must be a dominant homozygote (PP) and a recessive heterozygote (pp):
Parental generation: PP x pp
The first generation: Pp Pp Pp Pp
Pp represents a heterozygote.
If we cross these heterozygotes:
The first generation: Pp x Pp
The second generation: PP Pp Pp pp
If dominant allele determines the phenotype, there will be 3 plants (one PP and two Pp) with one phenotype and only 1 plant (pp) with another phenotype and vice versa.