Which innovative musical instrument, developed in the renaissance, used multiple quills fixed to a keyboard that would pluck pre-tuned strings when depressed?

Harpsichord

Explanation:

   The Harpsichord is the Piano's ancestor, so he is also a keyboard and string instrument. This instrument belongs to the group of clamped strings, that is, they generate the sound by tanging or pinching a string instead of percussing it as in the piano or clavichord. Given this fact it provides a very characteristic sound to our ears.

    Its earliest references date from the late 14th and early 15th centuries. It was widely used until the late eighteenth century, but in the following century fell into oblivion, having a flourish at the end of the nineteenth century. The musical period referring to the decades is the Baroque (approximately 1600 - 1750), where the Harpsichord was most used in the history of music appearing in great compositions by Johan Sebastian Bach.

   Nowadays the harpsichord can be found as a continuous instrument in chamber music and opera, but still especially when performing songs from the Baroque period.