The correct answer is: A. Double fertilization
Double fertilization is a unique process that occurs in flowering plants. It involves the fusion of two sperm cells with two different cells in the female gametophyte. The first sperm cell fertilizes the egg cell, forming the embryo. The second sperm cell fertilizes a central cell, forming the endosperm. The endosperm is a nutritive tissue that provides food for the developing embryo.
Haploid gametophytes are not unique to flowering plants. They are also found in mosses, ferns, and other non-flowering plants. Haploid gametophytes are the male and female reproductive structures of these plants. They are produced by meiosis, a type of cell division that reduces the number of chromosomes in a cell by half.
Pollen production is also not unique to flowering plants. It is found in all seed plants, including gymnosperms such as pines and firs. Pollen is a fine powder that is produced by the male reproductive organs of seed plants. It is used to fertilize the female reproductive organs, which are located on the same plant or on a different plant.
Therefore, the only option that is unique to flowering plants is double fertilization.