Dioecious plants have;

Dioecious and monoecious plants actually have something in common, in that they both bear unisexual flowers. This means that each bloom has only male or female reproductive parts. With dioecious plants, the male and female blossoms appear on separate plants. With monoecious plants, each plant has both male and female flowers.

Explanation:

With dioecious plants, the male and female blossoms appear on separate plants. With monoecious plants, each plant has both male and female flowers. By contrast, some plants have blooms that are bisexual. Each individual bloom has both male and female parts.

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.

the correct answer is: b. pantala flavescens, because these organisms would have a high level of genetic variation due to sexual reproduction.  

because involves two organisms (parents) whose genetic material combines, sexual reproduction leads to genetic variation in new generations of offspring. the offspring is an unique organism, with new genome, more protected with the higher ability to adapted. this process is fundamental to evolution.