Which tissue is formed by the union of sperm with two polar nuclei and provides nutrients to the embryo?

The endosperm is the tissue formed by the fusion of a sperm cell with two polar nuclei during double fertilization in angiosperms (flowering plants). This process results in a triploid cell that develops into the endosperm, which serves as a vital source of nutrients for the developing embryo within the seed. The endosperm provides essential carbohydrates, proteins, and fats that sustain the embryo during its growth until it can establish itself and begin photosynthesis once it germinates.

In contrast, other options such as the seed is the entire reproductive structure that includes the embryo, endosperm, and seed coat. The pollen tube serves as a conduit for sperm cells to travel toward the ovule, facilitating fertilization but is not involved in nutrient provision. Fruit refers to the mature ovary of a flowering plant, which may protect seeds and assist in their dispersal but does not directly provide nutrients to the embryo in the same way the endosperm does.