Rule reminder
In a cleft sentence, the FOCUSED element is the word or phrase that appears between 'è/sono' and 'che'. This is the new, important, or contrastive information. Example: È MARCO che ha chiamato → focused element = Marco (the subject). È AL BAR che ci siamo incontrati → focused element = al bar (place). Identifying the focus helps you understand the communicative intent of the sentence.
In 'È stata la nonna a fare la torta', what is the focused element?