The correct option is B (b)
Let’s examine the sentence: “The students were awaiting for the arrival of the chief guest.” The verb ‘await’ is a transitive verb. A transitive verb requires an object to complete its sense. This object can be provided by a noun, pronoun or gerund. In the sentence, however, ‘awaiting,’ which is the form of the verb ‘await,’ is followed by a preposition ‘for’ and not by any noun, pronoun or gerund. Hence, ‘awaiting’ does not have an object in the sentence and so be replaced by a word with similar meaning. Therefore, ‘waiting’ should be used in the place of ‘awaiting’ to correct it. Hence, error is in part b of the sentence. So, Option B is the correct answer.