The correct answer is D.
The sentence “I said to my brother, ‘Let us go to some hill station for a change'” is a suggestion. It is a polite way of asking someone to do something. In this case, the speaker is suggesting that they and their brother go to a hill station for a change.
Option A is incorrect because it is a command. The speaker is not ordering their brother to go to a hill station, they are suggesting it.
Option B is incorrect because it is a question. The speaker is not asking their brother if they would like to go to a hill station, they are suggesting it.
Option C is incorrect because it is a permission. The speaker is not giving their brother permission to go to a hill station, they are suggesting it.