If they ask you "debit or credit?" when you use the card at the store, that doesn't necessarily mean you have a choice, it just means they're asking which kind of card it is.
When you use a debit card to buy something, the money is transferred out of the cardholder's bank account immediately. So if there's no money in the account, the charge won't go through.
On the other hand, when you use a credit card to buy something, what happens is that the cardholder now owes that amount of money to the bank that issued the card. The sooner the cardholder pays it back, the less interest they have to pay (interest is a fee that's a percentage of the money owed - banks charge it because they need to profit from lending out money).
Most cards are *only* debit or *only* credit; if you try to use it the wrong way it won't go through. But I guess there may be some cards that can do both; in that case you probably should ask you Dad which way he prefers (because it could make a difference in how much it'll cost him).
P.S. It's actually most likely that this is just an ATM card, because it has a PIN. If there is no Visa or Mastercard logo on it, then it's an ATM card. An ATM card works like a debit card, but you can only use it in person and only at stores that let you use ATM cards. For cards like this, they'll never ask if it's debit or credit, because there's no Visa or Mastercard logo.