Each of these words is written in block capitals on a card. Which word looks exactly the same as itself in its mirror image, when the mirror is held at the side of the card so that left and right are swapped?

For the mirror image to read the same, every letter must itself be unchanged by a left-right flip, and the word must read the same backwards. In MOM the letters M and O are both unchanged by a side flip, and MOM reads the same backwards, so its mirror image is again MOM. BED, SUN and CAT contain letters such as B, E, S, N, C and A that either change shape or spoil the reversed reading.