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.$\newline$