Final answer:
The alternation of generations life cycle involves the coexistence of haploid and diploid multicellular stages in plants, algae, and some protists.
Step-by-step explanation:
The correct answer is D) Both haploid and diploid phases coexist. Alternation of generations refers to a life cycle in which an organism has both haploid and diploid multicellular stages. This type of life cycle is found in plants, algae, and some protists. In plants, for example, the life cycle alternates between the diploid sporophyte and the haploid gametophyte. The diploid sporophyte produces haploid spores through meiosis, which then develop into the haploid gametophyte stage. The gametophyte stage produces haploid gametes, which can fuse to form a diploid zygote and start the cycle again. Therefore, both haploid and diploid phases coexist in the alternation of generations life cycle.