Question

a purebred plant that has yellow flowers is crossed with a purebred plant that has white flowers. all the offspring have yellow flowers. which conclusion about yellow flowers is supported by these results?
yellow flower color is a dominant trait.
yellow flower color is a recessive trait.
yellow flower color is a heterozygous trait.
yellow flower color is a monohybrid trait.

Explanation:

In a cross between purebred yellow - flowered and purebred white - flowered plants, all offspring have yellow flowers. A dominant trait is one that is expressed in the offspring when crossed with a recessive - trait - carrying purebred. Since the yellow trait is expressed over white in the offspring, yellow is dominant. Recessive traits are masked in such a cross. Heterozygous refers to a genotype (not a trait classification here), and monohybrid refers to a cross involving one trait (not a trait type).

Answer:

Yellow flower color is a dominant trait.