Final answer:
During sexual reproduction, a homothallic mycelium typically contains haploid spores, which are produced when a diploid zygote undergoes meiosis.
Step-by-step explanation:
During sexual reproduction, a homothallic mycelium contains haploid spores. In the lifecycle of many fungi, including basidiomycetes and other fungal groups, sexual reproduction involves the fusion of two haploid hyphae to form a diploid zygote. This zygote then undergoes meiosis to produce haploid spores, which can germinate and grow into new haploid mycelia. These processes ensure genetic recombination and diversity among fungal populations. While the zygote is momentarily diploid, spores resulting from meiosis are haploid, which means the correct answer to the question is (b) Haploid spores.