The correct answer is: Endoderm
en·do·derm
the innermost layer of cells or tissue of an embryo in early development, or the parts derived from this, which include the lining of the gut and associated structures.
blas·tu·la
an animal embryo at the early stage of development when it is a hollow ball of cells.
mes·o·derm
the middle layer of an embryo in early development, between the endoderm and ectoderm.
ec·to·derm
the outermost layer of cells or tissue of an embryo in early development, or the parts derived from this, which include the epidermis and nerve tissue.
Hope this helped you! :3