The correct option is B Megaspore
A diploid megaspore mother cell is developed inside the nucellus. It then undergoes meiotic division to form four haploid megaspores. Out of these four megaspores, three gets degenerated and one remains functional. This functional megaspore then undergoes mitosis three times without undergoing cytoplasmic divisions. This results in the formation of 8 nuclei in the megaspore. These 8 nuclei then get organized into egg apparatus (consisting of an egg cell and two synergids) at micropylar end, a large central cell in the centre and three antipodal cells at chalazal end. This results in the formation of a 7−celled but 8−nucleate structure which is called the embryo sac.