The two ways of relating what a person has said are 1)direct and 2)indirect (reported).

In the direct speech, the exact words of the original speaker are repeated.

In the indirect speech (reported speech), the exact meaning of what the speaker told is expressed but not necessarily in the speaker’s exact words.

Option B is correct as the appropriate introductory verb \'could\' is used and the given direct speech is in the simple present tense. Therefore the sentence gets converted into simple past tense.

Option A is not in past tense, in option C third person form is not used, in option D appropriate conjunction \'if\' is missing. Therefore A,C, D are incorrect.

Option B is the correct answer.