Select the option that expresses the following sentence in reported (or indirect) speech.
The postmaster said to her, “You can purchase only two stamps at a time.”

Correct option is C

The correct expression in indirect speech is (c) "The postmaster told her that she could purchase only two stamps at a time."
Direct speech- Direct speech, also known as quoted speech or direct quotation, is a way of reporting someone's exact words.
Indirect speech- Indirect speech, also known as reported speech or indirect quotation, is a way of reporting someone's words or thoughts indirectly, without using their exact words.
Rules for converting direct speech to indirect speech-
· When the reporting verb of direct speech is in past tense then all the present tenses are changed to the corresponding past tense in indirect speech. 
· Reporting verbs such as ‘said/ said to’ changes to said, told.
· Comma inverted comma will be replaced with conjunction “That”.
· While changing direct speech to indirect speech, the modals used in the sentences change like: Can becomes could
· The  first person in direct speech  changes as per the subject of the speech.
· The  second person of direct speech  changes as per the object of reporting speech.
· The  third person of direct speech  doesn’t change.
This option correctly shifts the modal verb "can" to "could" to reflect the change from direct to reported speech, maintaining the informational intent of the statement without framing it as a question or an instruction.