Concept:A reason is valid only if it directly explains the event mentioned in the statement.
Explanation:The statement is about extra train services between City A and City B during a specific holiday period.
Reason (I) talks about flights between City A and City C. This is unrelated to train travel between City A and City B.
So, Reason (I) does not explain the decision.
Reason (II) states that many migrants from City B working in City A travel back to City B during October to January for festivals.
This directly explains the increased passenger demand and the need for special trains.
Hence, only Reason (II) is a possible reason.
Answer:Option C. Only II is a possible reason.