To solve this problem, we need to understand the development process of the functional megaspore in angiosperms.$\newline \bullet$In angiosperms, the process of megasporogenesis occurs within the ovule.$\newline \bullet$A diploid megaspore mother cell undergoes meiosis to produce four haploid megaspores.$\newline \bullet$Typically, only one of these megaspores becomes functional, while the others degenerate.$\newline \bullet$The functional megaspore undergoes mitotic divisions to develop into the female gametophyte, also known as the embryo sac.$\newline \bullet$The embryo sac is the structure that contains the egg cell and other cells necessary for fertilization and seed development.$\newline$Therefore, the functional megaspore in an angiosperm develops into the embryo sac.$\newline$This corresponds to Option 2: Embryo sac.