Final answer:
Polyploidy is rare in animals because triploid animals are usually sterile, leading to aneuploid cells that cannot produce viable offspring. In contrast, polyploidy in plants is common and can result in larger, more robust species.
Step-by-step explanation:
Polyploidy in animals is rare primarily because triploid animals are sterile, as meiosis cannot proceed normally with an odd number of chromosome sets. Consequently, this results in mostly aneuploid daughter cells that cannot produce viable zygotes. In animals, gametes are diploid, which, after combining with other gametes, restores the usual diploid state. However, polyploidy is common in plants and contributes to their diversity, with polyploid plants often being larger and more robust compared to their diploid counterparts. Therefore, the option (c) Diploid gametes most often combine with haploids, producing an unstable triploid most accurately explains why polyploidy in animals is rare, as these triploid organisms are typically sterile and cannot contribute to the gene pool.