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The chorion, one of the extraembryonic membranes in amniotes, originates from the somatic mesoderm (which lines the inner surface of the chorion) combined with the extraembryonic ectoderm (which forms the outer layer). However, the origin is often described as the fusion of the somatopleure (somatic mesoderm + extraembryonic ectoderm). Among the options, somatic mesoderm (D) is the most appropriate as it is the mesodermal component, while the ectodermal contribution is also essential. In strict germ layer terms, the chorion is derived from both extraembryonic ectoderm and somatic mesoderm.