To solve this problem, we need to analyze each statement about the prostomium of an earthworm:$\newline \bullet$(a) It serves as a covering for mouth.$\newline$ - The prostomium is a small, fleshy lobe that covers the mouth of the earthworm.$\newline$ - This statement is correct.$\newline \bullet$(b) It helps to open cracks in the soil into which it can crawl.$\newline$ - The prostomium aids the earthworm in burrowing by helping to push through soil and open cracks.$\newline$ - This statement is correct.$\newline \bullet$(c) It is one of the sensory structures.$\newline$ - The prostomium contains sensory cells that help the earthworm sense its environment.$\newline$ - This statement is correct.$\newline \bullet$(d) It is the first body segment.$\newline$ - The prostomium is not considered a true body segment; it is a pre-segmental structure.$\newline$ - This statement is incorrect.$\newline$Based on the analysis, the correct statements are (a), (b), and (c).$\newline$Therefore, the correct answer is Option 1: (a), (b), and (c) are correct.