In the following question, a sentence is given with a phrase highlighted in bold. Select the option given below that can replace the phrase in bold to make the sentence grammatically and contextually correct and mark that as your answer. If the sentence is correct as it is, then mark 'No correction required', as the answer. The company opted to revert back to traditional advertising methods after the experimental online campaign failed to yield satisfactory results.

(b) The original sentence contains an error related to redundancy. The phrase "revert back" is redundant because "revert" already implies going back to a previous state. Therefore, using "back" with "revert" is unnecessary. Option (b) is correct as it rectifies this error (it correctly removes the word 'back'). Options (a) and (d) do not rectify the original error. These options also use the redundant phrase 'revert back'. Option (c) is not correct because it contains a grammatical error. When using the constructions "planned to," "opted to," or similar expressions indicating a decision in the past, the verb that follows should be in the base form (infinitive). But in this option, the verb 'returned' (past tense form) is not in the base form. Hence, (b) is the correct answer. The correct sentence is: The company decided to revert to traditional advertising methods after the experimental online campaign failed to yield satisfactory results.