Step 1: Understanding the Concept:
We need to select the correct phrasal verb with 'call' to mean 'cancel'. The sentence is in the present perfect tense ("have not yet..."), so the past participle form of the verb is needed.
Step 2: Key Formula or Approach:
Let's define the phrasal verbs in the options:
- call off: To cancel an event or agreement.
- call for: To demand or require something. (e.g., "The situation calls for immediate action.")
- call at: To stop at a place for a short time (used for ships or trains). (e.g., "The train calls at all stations.")
- call in: To ask someone to come and help; to telephone a place such as a radio station.
Step 3: Detailed Explanation:
The sentence implies that the workers started a strike and have not yet cancelled it. The phrasal verb that means "to cancel" is "call off".
The verb form required after "have not yet" is the past participle. The past participle of 'call' is 'called'.
So the correct phrasal verb is "called off".
The sentence becomes: "The factory workers have not yet called off their strike."
Step 4: Final Answer:
The correct phrasal verb is 'called off'. This corresponds to option (A).