Concept:
Converting declarative statements containing modal auxiliaries from direct to indirect speech requires applying rules for backshifting modals alongside pronoun switches.
Step 1: Rules for conversion.
When the reporting verb is in the past tense ("said"):
• The coordinating conjunction
that is introduced to link the independent clauses.
• The first-person pronoun "I" shifts based on the subject of the reporting verb ("She"), thus transforming into
she.
• The modal verb
may undergoes a past-tense backshift, transforming into
might. The base verb following it ("buy") stays unchanged.
Step 2: Processing the transformation.
Let us assemble the components piece-by-piece:
• "She said," $\rightarrow$ "She said that"
• "I may buy" $\rightarrow$ "she
might buy"
• "a book on geography" $\rightarrow$ "a book on geography."
Combining these yields: "She said that she might buy a book on geography."
Step 3: Eliminating incorrect options.
• Option (A): Converts "may" to "will", which changes the conditional meaning entirely.
• Option (B): Changes the reporting verb to the present tense "says", which is incorrect.
• Option (C): Changes the simple modal aspect into a continuous form ("might be buying"), which alters the original aspect of the sentence.
• Option (D): Follows every grammatical instruction perfectly.
Therefore, Option (D) is the correct choice.