Final answer:
The lifecycle including both multicellular haploid and diploid stages is known as alternation of generations or haplodiplontic. It involves a haploid gametophyte stage producing gametes and a diploid sporophyte stage arising from the fertilization of gametes.
Step-by-step explanation:
The type of complex lifecycle that includes both multicellular haploid and multicellular diploid stages is best described as alternation of generations. This haplodiplontic life cycle is characterized by having two distinct forms: the haploid form known as the gametophyte, which produces gametes, and the diploid form called the sporophyte, which is formed when gametes fuse to form a zygote. The sporophyte produces haploid spores through meiosis that will develop back into gametophytes. This cycle is employed by some algae and all plants, creating a blend of haploid-dominant and diploid-dominant stages. In this type of lifecycle, meiosis is not directly involved in the production of gametes since the gametophytes are already haploid.