The reported speech is an example of a simple affirmative sentence but it is also a universal truth. Hence, the tense of the speech will not be converted but other rules, such as ‘said’ is converted into ‘told’ and ‘us’ is converted into them, are applicable. Option (a) is ruled out due to the conversion of the tense. Option (b) is ruled out due to the use of ‘us’ as it should be ‘them’. Option (c) is again ruled out due to the incorrect tense of the sentence.