When faced with the task of choosing the grammatically correct sentence from the given options, it is essential to consider the placement of adverbial phrases and the use of attributive adjectives. In the context provided, we have the following options and analysis:
"You should at once report it to the concerned authority." - While this sentence is understandable, "concerned authority" might imply an authority currently worried, rather than the intended meaning of 'relevant authority.'
"You should report it at once, to the authority concerned." - The placement of the comma before "to" disrupts the natural flow of the sentence.
"You should report it at once, to the concerned authority." - Similar to the previous option, the comma placement is unnecessary and disrupts clarity.
"You should at once report it to the authority concerned." - This option uses an atypical construct in English where the modifying adjective comes after the noun it modifies, commonly seen in legal or formal contexts. It accurately conveys "the authority that is concerned with this issue."
The most grammatically appropriate choice given the context is the last option, "You should at once report it to the authority concerned." This sentence maintains clarity, follows formal English conventions, and accurately conveys the intended meaning.