Final answer:
The common attribute between all flowering and nonflowering plants is the possession of the necessary structures for sexual reproduction. This includes the production of gametes and their subsequent fertilization, part of the alternation of generations cycle universal to plants. Option A is correct answer.
Step-by-step explanation:
Flowering and nonflowering plants, despite their differences, share a common form of reproduction. While flowering plants use bright and scented flowers to attract pollinators, nonflowering plants have other means of spreading their genetic material.
However, what unifies them is the necessary structures for sexual reproduction, specifically the production of gametes and fertilization leading to the diploid sporophyte generation. This cycle is known as the alternation of generations, which is a defining characteristic of the plant kingdom.
Moreover, both flowering and nonflowering plants can also have the capability for asexual reproduction, producing genetically identical clones of themselves through various means such as fragmentation or vegetative reproduction. Nonetheless, when considering the shared trait that all plants have for reproduction, it is their sexual structures and processes which are universal, not spores, pollen, or specific modes of asexual reproduction that can vary between plant groups.
The correct option that all flowering plants share with all nonflowering plants is thus (a) The necessary structures for sexual reproduction.