Identify the error part in the sentence: The spectators (a) / thronged the stadium to watch (b) / favourite star perform. (c) / No error. (d)

The error lies in part (c). The noun phrase 'favourite star' requires a determiner such as 'their' or 'the' before it. The correct phrasing would be 'their favourite star perform' since the spectators are watching their own favourite star. Without a possessive or definite article, the sentence is grammatically incomplete.