The correct answer is A. Let the window be opened.
The other options are incorrect because they do not follow the imperative mood. The imperative mood is used to give commands or instructions. In this case, the speaker is commanding the listener to open the window.
Option B, “Open window,” is in the imperative mood, but it is not a complete sentence. A complete sentence must have a subject and a verb. In this case, the subject is missing.
Option C, “Someone opens the window,” is in the indicative mood. The indicative mood is used to state facts or make predictions. In this case, the speaker is stating a fact that someone is opening the window.
Option D, “Window is opened,” is in the passive voice. The passive voice is used to focus on the action rather than the agent of the action. In this case, the speaker is focusing on the action of opening the window rather than the person who is opening it.
In conclusion, the correct answer is A. Let the window be opened.