Explanation:

- Hard Drive: Traditional hard drives are not solid-state. They use spinning disks to read and write data.

- CD: Compact Discs are optical storage media. They rely on physical discs to store data.

- DVD: Like CDs, DVDs are also optical storage media and not considered solid-state.

- USB Flash Drive: This is a solid-state storage device. It uses flash memory to store data with no moving parts.

Your answer is USB Flash Drive, which is correct, as it stores data using solid-state technology.