To convert direct speech into reported speech, we need to make several changes:

To convert direct speech into reported speech, we need to make several changes:

1. Change the reporting verb: "said" becomes "asked" for a question.
2. Remove quotation marks.
3. Change the tense: The past simple tense ("did you buy") shifts to the past perfect tense ("had bought").
4. Change pronouns: "you" becomes "I" (assuming the speaker is reporting what was asked of them).
5. Change demonstratives: "this" becomes "that".
6. Change the sentence structure from interrogative to assertive (subject + verb order, no question mark).

Let's apply these rules to the original sentence: "Mr. Singh said, 'Where did you buy this car?'"

• Reporting verb: "said" changes to "asked".
• Question word: "where" remains.
• Tense change: "did you buy" (past simple) becomes "I had bought" (past perfect).
• Pronoun change: "you" becomes "I".
• Demonstrative change: "this car" becomes "that car".

Now let's evaluate the given options:

• (A) Mr. Singh has asked if I can tell him where did I buy this car.

  • Incorrect tense for the reporting verb ("has asked" instead of "asked").
  • "if I can tell him" is unnecessary and changes the meaning.
  • "did I buy" is still in past simple and interrogative form.
  • "this car" should be "that car".

• (B) Mr. Singh asked where did I buy this car.

  • "did I buy" is still in past simple and interrogative form.
  • "this car" should be "that car".

• (C) Mr. Singh asked where I had bought that car.

  • "asked" is correct.
  • "where" is retained.
  • "I had bought" correctly shifts the tense to past perfect and uses the assertive structure.
  • "that car" correctly changes from "this car".
  • This option follows all the rules for reported speech.

• (D) Mr. Singh was asking where I had bought that car.

  • "was asking" (past continuous) is grammatically possible but "asked" (past simple) is a more direct and common conversion for "said" when reporting a single question.

Therefore, option (C) is the most accurate conversion.

The final answer is $\boxed{\text{C}}$