Megasporium is equivalent to

The correct option is ovule.

In gymnosperms and angiosperms (seed-bearing plants), the terms used for reproductive structures are homologous to those used in lower plants (like bryophytes and pteridophytes). Understanding these structural equivalencies helps in tracing evolutionary relationships.

Let us break down the terminology to understand why the ovule is equivalent to the megasporangium (often referred to as megasporium in some simplified contexts):
1. Megasporangium: A sporangium is a structure in which spores are produced. A megasporangium is a female sporangium that produces megaspores.
2. Ovule: In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of the megasporangium (specifically called the nucellus in seed plants) protected by one or two layers of integuments.
3. Therefore, morphologically and functionally, the ovule of seed plants is equivalent to an integumented megasporangium (or megasporium).

To clarify the other options:
- Embryo sac: The embryo sac represents the female gametophyte, which develops from the functional megaspore inside the megasporangium.
- Fruit: The fruit is a mature, ripened ovary, which develops after fertilization.
- Nucleus: The nucleus is a cellular organelle containing genetic material, whereas the central nutritive tissue of the ovule is called the nucellus (not to be confused with nucleus).