Which spore is thick-walled and diploid?
Zygospore results from conjugation of progametangia of compatible mating types, undergoes karyogamy becoming transient diploid (2n) nucleus within thick, melanized, warty wall containing multiple layers of sporopollenin-like material conferring resistance to UV, desiccation and microbial attack. Prior to germination meiosis reduces ploidy producing haploid sporangiospores capable of germination. Conidia, ascospores, basidiospores are typically haploid and thin-walled adapted for wind dispersal, not diploid survival. Zygospore therefore represents only diploid thick-walled resting spore in early fungal groups like Rhizopus, allowing survival through adverse seasons.
Ref: Campbell Biology 12th ed., Chapter 31: Zygospore Thick-Walled Diploid Resting; NCERT Class 11 Zygospore Dormancy; NCBI Zygomycete