To solve this problem, we need to analyze the given statements:$\newline \bullet$Statement I: Decomposition is a process in which the detritus is degraded into simpler substances by microbes.$\newline$ - This statement is correct. Decomposition involves the breakdown of organic matter by microorganisms, resulting in simpler substances.$\newline \bullet$Statement II: Decomposition is faster if the detritus is rich in lignin and chitin.$\newline$ - This statement is incorrect. Lignin and chitin are complex and resistant compounds, which slow down the decomposition process.$\newline$Now, let's determine the correct option based on the analysis:$\newline \bullet$Statement I is correct, but Statement II is incorrect.$\newline$Therefore, the correct answer is Option 3: Statement I is correct but Statement II is incorrect.