Which of the following is a tiny cell used by ferns and fungi to reproduce?

The correct answer is that a spore is a tiny cell used by ferns and fungi to reproduce. Spores are a fundamental part of the reproductive lifecycle for both ferns and many types of fungi. These reproductive cells are capable of developing into a new organism without the need for fertilization, allowing them to disperse into various environments where they can grow into mature plants or fungi. Ferns produce spores in structures called sporangia, often found on the undersides of their fronds, while fungi typically release spores from specialized structures called fruiting bodies.

The other options, while they are all related to reproduction in different organisms, do not fit the specific context of ferns and fungi. Gametes are involved in sexual reproduction and require two cells to combine to form a new organism. An ovum is a type of gamete specifically in animals, and embryos are the early developmental stages of fertilized eggs. Thus, the unique reproductive strategy of ferns and fungi relies on spores, making this the appropriate choice.