Earthworm

Let's analyze each of the given statements about the prostomium of an earthworm to determine their accuracy:

1. Statement (a): "It serves as a covering for mouth."
   • The prostomium is a small, protruding lobe located at the anterior end of the earthworm, in front of the mouth. It acts as a covering for the mouth, protecting the entrance while the earthworm moves through soil. Therefore, this statement is correct.
2. Statement (b): "It helps to open cracks in the soil into which it can crawl."
   • The prostomium indeed plays a role in the earthworm's burrowing activity. It helps to probe and thrust into the soil, facilitating the earthworm's movement by opening up cracks in which the earthworm can crawl. Thus, this statement is correct.
3. Statement (c): "It is one of the sensory structures."
   • The prostomium is equipped with sensory functions, enabling the earthworm to sense its environment. It is involved in sensing light and chemical signals in the soil. Therefore, this statement is also correct.
4. Statement (d): "It is the first body segment."
   • The prostomium is not considered an actual body segment of the earthworm; rather, it is a pre-segmental structure located ahead of the first true body segment. Hence, this statement is incorrect.

Based on the detailed analysis of each statement, we can conclude that statements (a), (b), and (c) are correct. Therefore, the correct option is:

(a), (b) and (c) are correct