The male and female reproductive parts in plants are different in terms that the female plants (pistiliate) possess a functional pistil but lack stamens, while the male plants (staminate) contains stamens, but no pistils. Therefore the complete flower structure have stigma, style, pistil, stamen and the other acessory parts of the reproductive system, while a imperfect (pistillate) flowers don't have a stamen. According to the question and the information above the correct alternative to differenciate the gender of that plant flower is A. stamen.