The correct answer is: A. The Policemen keep all vulnerable places under constant vigil.
The other options are incorrect because they do not accurately reflect the tense of the sentence. The sentence states that all vulnerable places are kept under constant vigil by the Policemen. This means that the action of keeping the places under constant vigil is ongoing and has been happening for some time. The other options suggest that the action is either about to happen or will happen in the future.
Option B, “The Policemen are keeping all vulnerable places under constant vigil,” is incorrect because it uses the present progressive tense. The present progressive tense is used to describe actions that are happening at the moment or that are happening repeatedly over time. In this case, the action of keeping the places under constant vigil is not happening at the moment, and it is not happening repeatedly over time. It is an ongoing action that has been happening for some time.
Option C, “The Policemen will be keeping all vulnerable places under constant vigil,” is incorrect because it uses the future progressive tense. The future progressive tense is used to describe actions that will be happening at a specific time in the future or that will be happening repeatedly over time in the future. In this case, the action of keeping the places under constant vigil is not happening at a specific time in the future, and it is not happening repeatedly over time in the future. It is an ongoing action that has been happening for some time.
Option D, “The Policemen will keep all vulnerable places under constant vigil,” is incorrect because it uses the future tense. The future tense is used to describe actions that will happen at a specific time in the future. In this case, the action of keeping the places under constant vigil is not happening at a specific time in the future. It is an ongoing action that has been happening for some time.