Choose the grammatically correct sentence:-

The correct answer is Option (4) → Reema said that she didn't believe me.

Reported statement after a past reporting verb:

• Reporting verb: "said" stays, and the reported words are joined by that (which may be left out).
• Tense change: after said, the tense moves back: "don't believe" becomes didn't believe.
• Pronoun changes: "I" becomes she, and "you" becomes me, to suit the reporter.
• This sentence: Reema said, "I don't believe you." → Reema said that she didn't believe me.
  Example: Ravi said, "I don't like tea." → Ravi said that he didn't like tea.

Why the Other Options are Incorrect?

Option 1: Reema said I didn't believe. — Incorrect.

• "I" now seems to refer to the reporter, not to Reema.
• The object of "believe" is missing.

Option 2: Reema said she don't believe. — Incorrect.

• "She don't" breaks agreement (she doesn't), and the tense is not moved back after "said".
• The object of "believe" is missing.

Option 3: Reema said I don't believe you. — Incorrect. These are Reema's exact words without inverted commas; neither the pronouns nor the tense have been changed.