A cuboid has six sides of different colours. The red side is opposite to black. The blue side is adjacent to white. The brown side is adjacent to blue. The red side is face down. Which one of the following would be the opposite to brown?

To find the opposite of brown, we first identify the pairs of opposite sides. The problem states that red is opposite to black. Since there are six sides in total, four sides must be adjacent to the red-black axis. These four sides are blue, white, brown, and the remaining sixth color.

The problem states that blue is adjacent to white and brown is adjacent to blue. In a cuboid, any given side is adjacent to four sides and opposite to only one. Since blue is adjacent to red, black, white, and brown, its opposite must be the sixth color.

Crucially, in the ring of four sides surrounding the red-black axis, each side is adjacent to two others in that ring and opposite to one. If white is adjacent to blue and brown is also adjacent to blue, then white and brown must be on either side of blue. Because the red and black sides are already occupied as a pair, and blue is adjacent to both white and brown, the only side left for brown to be opposite to is white. Therefore, white is the side opposite to brown.