Select the most appropriate option to fill in the blank.

The garret where he lay at night had long been ______, and a great number of mice had made their home in it.

vacant

The correct option to fill in the blank is vacant.

The sentence is about a garret that had been empty for a long time. The word "vacant" means empty or unoccupied, which is the most appropriate word to describe the garret in the sentence.

The other options are not as appropriate.

• Partial means incomplete or partly filled. This does not make sense in the context of the sentence because the garret is described as being completely empty.
• Visible means able to be seen. This does not make sense in the context of the sentence because the garret is described as being hidden or out of sight.
• Thick means having a relatively large distance between opposite sides. This does not make sense in the context of the sentence because the garret is not described as being physically large.