The correct option is B a police officer
Let’s examine the sentence: “There was a police standing at the corner of the street.” The mention of ‘police’ does not indicate anything and prevents sentence from making any sense. Just ‘police’ can mean ‘police officer’ or ‘police van’ or ‘police station’ or anything else related to police. So the sentence a to specifically state what ‘police’ means here. Now, the sentence states that police-related entity was ‘standing at the corner of the street,’ and so we can assume that it would be a police officer only. Option D cannot be used here as the sentence is using verb of singular noun and ‘policemen’ in this option is plural noun. Option A cannot be used as the clause ‘standing at the corner of the street’ cannot be used for a police station. Hence, Option B is the correct answer.