Final answer:
Sentence (c) lacks proper parallelism and could be revised to "During his stay at the Ospedale della Pieta, Vivaldi was appointed as the master violinist" to maintain a similar structure with the other sentences.
Step-by-step explanation:
The sentence that lacks proper parallelism in the paragraph above is sentence (c) "He was appointed to the level of master violinist during his stay at the Ospedale della Pieta." To correct the lack of parallelism, the sentence could be revised to match the structure of the other sentences more closely, such as "During his stay at the Ospedale della Pieta, Vivaldi was appointed as the master violinist." This revised sentence achieves parallelism by maintaining a similar structure to the others: subject + verb + complement.