Cubes surrounded in a 125-cube arrangement

A cubical block assembled from 125 small identical cubes has an edge length of √[3]{125} = 5 cubes along each side.

Cubes that are "surrounded by other cubes from each side" are the inner, completely unexposed core cubes. These are the interior cubes left behind when you strip away the outer single layer of cubes from all 6 faces of the large structure.

Stripping 1 layer from both ends reduces each side length from 5 to 5 - 2 = 3 cubes. The volume of this remaining inner core block is: 3 × 3 × 3 = 27 cubes.

Answer: (a).