The smallest egg belongs to-
a. Mammals
b. Fishes
c. Amphibians
d. Reptiles

Hint: The egg of this organism is small, about the size of a marble. Organisms in their culture may have similarly laid eggs earlier, a characteristic that they inherited from their other lower-group ancestors.

Complete answer:
> Option A is correct. Both oviparous eggs usually lay externally. Viviparous produces an egg internally, and the development of that egg into the foetus, embryo, zygote occurs internally only and then the young ones are born. The size of a mammal's egg is the size of a feather. Monotremes, along with placentals (Eutheria), and marsupials (Metatheria), are one of the three major groups of living mammals. Rather than bearing live young, they lay eggs, just like all mammals, the female monotremes feed their young with milk.

> Option B is incorrect. Fish roe is the term used for female fish eggs and is also known as sturgeon roe. It is the fully mature inner ovarian masses, or the produced outside ovarian masses of fish and other marine animals, such as shrimps, scallops, sea urchins, and squid. Roe is used as a seafood in many recipes as a cooked ingredient, and as a raw ingredient. It is one of fat-soluble vitamin D sources that works along with omega-3 fats.

> Option C is incorrect. Unlike other vertebrate tetrapod’s (reptiles, birds, and mammals), amphibians do not develop amniotic ovaries. They must then lay their eggs in water, so that they would not dry out. Their eggs are generally coated in a liquid resembling jelly.

> Option D is incorrect. Many reptiles use smooth, leathery shells to lay eggs, but minerals in the shells can make them tougher. Crocodilians and other types of turtles lay eggs much like a bird's egg with tough shells. In certain instances, female reptiles build nests to shield their eggs before they are ready to hatch.

Hence, The correct answer is option (A).

Note: The presence of vitellogenin genes (a protein that is needed to shape egg shells) is shared with birds; the development of this symplesiomorphy suggests it is the monotreme common ancestor, marsupials, and placental mammals was oviparous, and that this feature was preserved in monotremes but lost in all other current mammal groups.