Verified answer

They reproduce sexually.

true but not the reason.

They exhibit only one form of a trait.

Since they are using "purebred", I assume they mean a plant that breeds true like a pure homozygous individual either dominant or recessive. That would make this the correct answer even though the reason they are useful is that peas have a variety of alleles for each trait he studied. The purebred is apparently meant to mean just the true breeding plants. Those only exhibit one form of a trait.

They have a variety of traits.

That is true for peas in general but it doesn't really speak to being purebred. I assume it isn't the best answer because purebred implies one form of trait being expressed consistently. It is confusing because purebred isn't defined exactly since peas have many different traits.

They reproduce asexually.

Not relevant and it isn't the normal mode of reproduction in peas..