Explanation:

Snake:

Snakes are reptiles that have a long, cylindrical body covered in scales. They have muscles and bones just like other animals, but they move in a unique way - by contraction and expansion of their body. This type of movement is achieved through the coordinated action of their muscles.

Contraction and Expansion:

Snakes move by contracting and expanding their body in a process called "serpentine locomotion." They have a large number of vertebrae, which allows them to have a flexible spine. By contracting and expanding the muscles along their body, snakes are able to push against the ground and propel themselves forward.

Advantages of Serpentine Locomotion:

This mode of movement provides several advantages to snakes:

1. Adaptability: The ability to move by contraction and expansion allows snakes to navigate through various terrains, including narrow crevices and dense vegetation.

2. Efficiency: Snakes are able to move quickly and with minimal energy expenditure using serpentine locomotion. This is particularly useful when hunting or evading predators.

3. Stealth: The smooth, undulating motion of snakes makes them difficult to detect. Their ability to move silently allows them to sneak up on prey or escape from potential threats.

4. Climbing: Snakes can also use serpentine locomotion to climb trees or other structures by gripping onto rough surfaces with their scales and using their muscles to propel themselves upward.

Comparison with Other Options:

a) Snail: Snails move by crawling using their muscular foot, but they do not have muscles and bones that allow them to contract and expand their body for movement.

c) Earthworm: Earthworms also crawl using their muscles, but they do not have bones or the ability to contract and expand their body for movement.

d) Crab: Crabs have a hard exoskeleton and jointed limbs that allow them to scuttle sideways, but they do not move by contraction and expansion of their body.

In conclusion:

Among the given options, the snake is the only one that has muscles and bones and moves by contraction and expansion of its body. This unique mode of movement allows snakes to navigate various environments, hunt effectively, and escape from potential threats.