Select the option that expresses the given sentence in active voice.
The letter was being typed by the secretary when the phone rang.

Correct option is C

 The correct active voice of the given sentence is  The secretary was typing the letter when the phone rang.
Explanation: To convert from passive to active voice:
· The subject of the passive sentence ("The letter") becomes the object in the active voice.
· The doer of the action ("The secretary") becomes the subject.
Structure:
· Passive: [Object + was/were being + past participle + by + subject].
· Active: [Subject + was/were + verb(-ing) + object].
Example:
· Passive: The book was being read by her.
· Active: She was reading the book.