You're right, but the question is asking about a specific rule and is asking you to assume that the four cards presented are an accurate representation of the rest of the deck. "If a vowel is printed on one side of the card, then an even number is printed on the other side."

Therefore, we can prove the rule by using two specific cards.

Flip over the "U" card because it directly matches the variables given. U = vowel. A vowel is therefore printed on the card, and thus the other side of the card must have an even number printed on it, else the rule is false. You can prove the rule absolutely false with this card because if the character on the other side of the card isn't an even number, the rule is false. You know this because this card has a vowel without an even number on the opposing side. However, if the character is an even number, you've only proven the rule true in this instance. There could still be another card that breaks the rule.

In order to unequivocally state that the rule is true, you have to then turn over a card with an odd number on it, because you've already proved that a vowel will be paired with an even number. Now you have to make sure that vowels will not be paired with an odd number as well. If they were, then the rule would be false. Therefore, you have to turn over a card with an odd number in order to determine what odd numbers will be paired with. Again, if it's a vowel, the rule is false and if it's a consonant, the rule is true.

The reason we wouldn't use the "J" card is because we don't care what consonants are paired with. The rule doesn't state a relationship between consonants and anything else, so we learn nothing by turning the card over.

The reason we chose "U" instead of "2" is because we need to be sure that an even number is printed on the other side of the "U" card. We don't necessarily care what even numbers are paired with as long as every vowel is paired with an even number.