Four cards lie on table. Each card has a number printed on one side and a colour on the other. The faces visible on the cards are $2,3,$ red, and blue.

Proposition: If a card has an even value on one side, then its opposite face is red.

The card which MUST be turned over to verify the above proposition are

1. $2,$ red
2. $2,3,$ red
3. $2,$ blue
4. $2,$ red,blue