Answer:

When someone is setting off on a journey, it is customary to wish them well and express good wishes for their safe and successful trip. In English, the most common phrase used in this situation is "bon voyage," which is option A in the given choices.

Explanation:

- Bon Voyage:
The phrase "bon voyage" is a French expression that directly translates to "good journey" or "have a good trip." It is a traditional way of wishing someone well as they embark on a journey. The use of this phrase adds a touch of elegance and sophistication to the well-wishing. It is widely recognized and used in English-speaking countries as a way to bid farewell to travelers.

- Check In:
"Check in" is not the appropriate phrase to use when someone is setting off on a journey. It is a term commonly used in the context of air travel, where passengers are required to check in at the airport before boarding their flight. However, it does not convey the same sentiment of well-wishing as "bon voyage" does.

- Take Off:
"Take off" is a phrase used to describe the action of an aircraft leaving the ground and ascending into the air. While it is a relevant term in the context of air travel, it is not the appropriate phrase to use when bidding farewell to someone who is starting a journey. It focuses more on the act of departure rather than expressing good wishes for the journey itself.

- Sail:
"Sail" is a verb that specifically refers to traveling by boat or ship using sails to harness the wind. While it is a suitable term for someone embarking on a sea voyage, it does not encompass all types of journeys. Additionally, it is not as commonly used in everyday conversation as "bon voyage."

In conclusion, the correct phrase to use when a person is setting off on a journey is "bon voyage." It conveys well-wishing and good intentions for the traveler's journey, regardless of the mode of transportation or destination.