Top-shaped multiciliate male gametes and the mature seed which bears only one embryo with two cotyledons, are characteristic features of:

The characteristic features of top-shaped multiciliate male gametes and the mature seed which bears only one embryo with two cotyledons are found in cycads. Cycads are a group of seed plants that are characterized by the presence of motile sperm and the production of seeds with a single embryo and two cotyledons. So, the correct answer is option D. Notice that the other options are not correct. Polypetalous angiosperms are flowering plants with separate petals, gamopetalous angiosperms are flowering plants with fused petals, and conifers are a group of seed plants that produce seeds in cones.