Final answer:
Most modern plants spend the majority of their life cycle in the diploid phase as sporophytes, which has become increasingly predominant throughout plant evolution.
Step-by-step explanation:
The majority of plants recognized today spend the majority of their life in the diploid phase.
Plants undergo alternation of generations, which means that their life cycle includes both haploid and diploid stages. However, throughout the evolutionary history of plants, there has been a trend towards greater prominence of the diploid stage, known as the sporophyte. Early plants, like mosses, spent most of their lifecycle as haploid gametophytes, but most modern plants such as flowering plants and conifers, spend the larger part of their lifecycle as diploid organisms. This is thanks to the development and predominance of the sporophyte stage in the lifecycle. During this phase, the plant grows, matures, and forms the structures that will produce the gametes or reproductive cells involved in the continuation of the species.
Within the sporophyte phase, cells undergo meiosis to produce haploid spores, which will then develop into new, haploid gametophytes. The gametophytes produce gametes, and when two gametes fuse, they form a diploid zygote, which develops into the sporophyte, and the cycle continues. Over time, the sporophyte phase has grown increasingly dominant, and in many plants today, this stage is the most visible and long-lived part of the lifecycle, commonly taking the form of the plants we see around us, such as trees and flowering plants.