Concept:A 3:1 offspring ratio in a cross indicates a Mendelian monohybrid cross, where one allele is dominant and the other is recessive.Explanation:When yellow-seeded plants are cross-bred and produce yellow and white seeds in a 3:1 ratio, both parents must have been heterozygous for the seed colour gene.The white trait appears only in the homozygous recessive form, while yellow appears in both homozygous and heterozygous forms.Since the yellow trait masks the white trait and appears in three out of four offspring, yellow seed colour is dominant.Answer:D. a dominant trait