Final answer:
The featured string instrument in Paganini's 24 Caprices is the violin, known for its expressive capabilities that match the virtuosic requirements of the compositions.
Step-by-step explanation:
The string instrument featured in Paganini's 24 Caprices is the violin. Paganini's 24 Caprices are a series of solo violin pieces that are known for their technical difficulty and virtuosic demands on the performer. The violin is a bowed string instrument with a high pitch and is capable of a wide range of dynamics and expressive techniques, making it the ideal instrument for these caprices.