Question

how did nightingale fall a prey to the heartless frog?

Answer 1

It was because of her

Innocence
Gullibility

Answer 2

The frog was a hated singer in the bog. He had a rough voice and the creatures living in the bog loathed his  voice, still he kept on singing.

But with the arrival of the nightingale the situation changed. His position was  challenged. So, he posed as a great singer and offered to teach the nightingale.

The nightingale, being foolish,  accepted him as her teacher and tried to imitate his voice. With the result, her own voice became hoarse and  uninspired. Other creatures also became tired of listening to her song.

At this, the frog scolded her. In order to  please the frog, she rehearsed a lot which ended in her bursting a vein and she died.