In yeast, the sexual reproduction is by conjugation. After fusion, the zygotic nucleus divides meiotically and forms haploid spores. The unicellular structure with haploid spores is

The correct option is Gametangium.

Here is the step-by-step educational explanation:
1. Conjugation in Yeast: During sexual reproduction in yeast, two vegetative haploid cells of compatible mating types behave directly as unicellular sex organs, known as gametangia.
2. Fusion: These two cells undergo conjugation, where their cell walls fuse at the point of contact, followed by the fusion of their cytoplasm (plasmogamy) and nuclei (karyogamy). This fusion results in the formation of a diploid zygotic nucleus.
3. Meiosis and Spore Development: The diploid zygotic nucleus undergoes meiosis to produce four haploid nuclei. Each of these nuclei, along with some surrounding cytoplasm, develops a protective wall to become a haploid spore.
4. Structure: Since the unicellular conjugating cells themselves act as the gametangia that fuse and ultimately contain these haploid spores, the unicellular structure associated with this process is the gametangium.