Convert the given sentence into indirect speech:

My mother said, "Cleanliness is next to godliness."

In the given statement, the actual words of the speaker are mentioned as it is. It is known as direct speech. We need to convert it into indirect speech, as reported by someone else. If the reporting verb is in the past tense, then while converting it into indirect speech, the sentence will also change into the corresponding past tense. However, there is an exception to this case. If the reported speech is about a universal truth or a habitual fact, then the tense will remain unchanged. Cleanliness is next to godliness is a habitual fact. Hence, its tense should remain unchanged. So, option C is the correct answer. Options A and D are in the present and future tenses. Option B uses quotation marks which indirect speech does not use. Hence, these can be eliminated.