Let us travel . . . . . . . . night, it will be cool then. A. by B. at C. in D. during

The correct answer is: at.

The word “at” is used to indicate a specific time or point in time. In this case, the speaker is suggesting that they travel at night, when it will be cooler.

The other options are not as appropriate in this context. The word “by” is used to indicate a means of transportation. The word “in” is used to indicate a period of time or a location. The word “during” is used to indicate a specific period of time within a larger period of time.

Here are some examples of how the other options could be used in a sentence:

• We traveled by car to the beach.
• We stayed in the hotel for two nights.
• The party was during the summer.