The endosperm of gymnosperms is a pre-fertilization tissue that is equivalent to the female gametophyte.
The endosperm of gymnosperms is haploid. It is a pre-fertilization tissue that is equivalent to female gametophyte, therefore it is haploid in nature. However, in angiosperms, it is the post-fertilization tissue and is triploid in nature.
Therefore, the correct option is D) Haploid.
There are series of events that happen in the development of plants after their fertilization process to form a fruit from a diploid zygote. The four different developmental steps that occurs during post fertilization are: