Answer: Option C
In the sentence, the phrase "it will be cool then" implies that the speaker is referring to a particular season or time of day. We can use the preposition "in" to refer to a time period during which something happens.
Here are some rules and examples of how to use the preposition "in":
• "In" is used to refer to a period of time within a day, month, year, or season.Example: She was born in December.
• "In" is used to refer to a time of day.Example: I like to take a walk in the morning.
• "In" is used to refer to the future or a time period.Example: We are going to have a meeting in a week.
Therefore, in the given sentence, the phrase "Let us travel in night" means that the speaker is planning to travel during the night, when it is cooler.
Option A, "at", is incorrect because it generally refers to a specific time.Example: I will meet you at 3 pm.
Option B, "by", is incorrect because it means "no later than" or "before a specified time".Example: We need to finish this project by Friday.
Option D, "during", is incorrect because it generally refers to an activity that happens throughout a period of time.Example: We played games during the party.
Option E, "none of these", is not the correct answer because "in" is the appropriate preposition to use in the given sentence.
Therefore, the correct answer is option C, "in".