What would be the ploidy of the endosperm of a seed resulting from a diploid female plant and a tetraploid male plant?

The correct option is A tetraploid
During sexual reproduction in flowering plants, when one male gamete fuses with the 2 polar nuclei of the embryo sac, it gives rise to the primary endosperm nucleus (PEN). When the parents are diploid, then both the gametes are haploid and the two polar nuclei are also haploid. But when the male parent is tetraploid, the male gamete will be diploid. So, the fusion of a diploid male gamete with 2 haploid polar nuclei will give rise to a tetraploid endosperm.