The Mistress said, “Fanny, did you mail the two letters I gave you yesterday?”

The mistress enquired of Fanny if she had mailed the two letters she gave her the day before.

The mistress enquired of Fanny whether she had mailed the two letters she had given her yesterday.

The mistress asked Fanny if she had mailed the two letters she had given her the day before.

None of these

The mistress enquired of Fanny if she had mailed the two letters she gave her the day before.

The reporting verb "enquired" changes the direct speech into indirect speech.

The past tense verb "gave" and the time word "yesterday" correctly change to "the day before" in indirect speech.