The correct option is C 3
In a dice, the sum of numbers on opposite faces is always 7.
In the given net, face with number 4 will go behind to face with number 5.
Also, the face with question mark will go behind to the face with the number 5.
So 4 and ? are the numbers of opposite faces.
that means 4 + ? = 7
? =3
So the missing number is 3