Bobolinks are small songbirds that are known for their melodious singing. They belong to the blackbird family and are found in North America. Bobolinks are highly migratory birds and spend their breeding season in the northern parts of the continent and then migrate to South America for the winter.

Bobolinks are known for their distinctive appearance during the breeding season. The male bobolinks have a black coloration on their head, back, and breast, and a white coloration on their wing coverts and rump. They also have a yellow patch on the back of their neck. On the other hand, the female bobolinks have a dull brown coloration, with streaks on their breast.

Now let's discuss the options given in the question and why option 'B' is the correct answer:

a) They flew away:
Bobolinks are indeed capable of flying away, like any other bird. However, this option does not specifically mention any action or behavior specific to bobolinks.

b) They sang melodiously:
This is the correct answer. Bobolinks are known for their beautiful and melodious songs. The male bobolinks sing complex and unique songs during the breeding season to attract mates and defend their territories. Their songs are often described as bubbly, tinkling, or metallic in nature.

c) They hid in the hills:
There is no specific behavior or evidence to suggest that bobolinks hide in the hills. Bobolinks are typically found in open grasslands, meadows, and fields during the breeding season.

d) They slept on the stiles:
There is no evidence or behavior to suggest that bobolinks sleep on stiles. Bobolinks usually sleep in dense vegetation or trees, where they can find shelter and protection.

In conclusion, the correct answer is option 'B' - Bobolinks sang melodiously. They are known for their beautiful songs during the breeding season, which play an important role in courtship and territorial defense.