Which term refers to an undeveloped ovum?

The term that refers to an undeveloped ovum is "oogonium." An oogonium is a female gametangium that produces ova, or eggs, through the process of meiosis. In many organisms, including various algae and fungi, the oogonium is the structure where the initial stages of egg formation occur. It plays a crucial role in sexual reproduction, as it is the precursor to the mature ovum that can be fertilized by male gametes.

The remaining terms have distinct meanings: a zygospore is a thick-walled resting spore formed from the zygote of certain fungi and algae, an antheridium is the male reproductive structure that produces sperm in plants and algae, and a pyrenoid is a protein body often found in algae, associated with starch storage and photosynthesis. Thus, "oogonium" is the only term correctly identifying an undeveloped ovum.