What is the young one of frog called as?

Tadpole

The young one of a frog is called a tadpole.

A tadpole is the immature stage of a frog or toad. It has a long tail and gills, and it lives in water. As the tadpole grows, it loses its tail and gills, and its lungs develop. It eventually emerges from the water as a frog or toad.

The other options are incorrect. A toad is an adult frog, not a young frog. A cygnet is the young of a swan, not a frog. A kid is the young of a goat, not a frog.