Final answer:
The most popular and widely used stringed keyboard instrument from the 1400 through the mid-1700s was the harpsichord.
Step-by-step explanation:
The most popular and widely used stringed keyboard instrument from the 1400 through the mid-1700s was the harpsichord. The harpsichord was a keyboard instrument that produced sound by plucking strings with quills, whereas the organ produced sound through the vibration of air in pipes. The clavichord was another keyboard instrument, but it was not as popular as the harpsichord during this time period. The accordion, on the other hand, was not invented until the 19th century, making it an incorrect option.