The flightless bird from the given options is the Penguin.

Explanation:
Penguins are a group of flightless seabirds that are highly adapted for life in the water. They are known for their unique appearance and behavior, and are found primarily in the Southern Hemisphere, particularly in Antarctica. Let's compare the given options to understand why the correct answer is Penguin.

Sparrow:
Sparrows are small passerine birds that are known for their ability to fly. They have wings and are capable of sustained flight. Therefore, they are not flightless birds.

Cock:
Cocks, also known as roosters, are male chickens. They are capable of flying short distances, although their flight is not as strong or sustained as that of other bird species. However, they are not considered flightless birds.

Crane:
Cranes are large, long-legged birds that are known for their impressive flying abilities. They have powerful wings and are capable of long-distance flights. Therefore, they are not flightless birds.

Penguin:
Penguins, on the other hand, are flightless birds that have evolved for life in the water. They have streamlined bodies, flipper-like wings, and webbed feet, which make them excellent swimmers. While they are not capable of sustained flight, they are highly adapted for swimming and diving in the ocean. Penguins use their wings as flippers to propel themselves through the water, where they catch fish and other marine creatures for food.

In conclusion, the flightless bird from the given options is the Penguin.

Penguin