When two goods are completely interchangeable, they are referred to as perfect substitutes. This means that consumers perceive these goods as having equal value and are willing to substitute one for the other without any preference or loss in satisfaction.

Option 1: Perfect substitutes means that the goods are identical in the eyes of consumers, and they can be used interchangeably without any change in satisfaction or utility.

Option 2: Perfect complements, on the other hand, are goods that are used together in fixed proportions. They are consumed together and cannot be substituted for each other.

Option 3: Giffen goods are a type of inferior goods where an increase in price leads to an increase in demand. This is a rare exception to the law of demand and is not related to the concept of interchangeable goods.

Option 4: Veblen goods are goods that are perceived as prestigious or high-status, and their demand increases as their price increases. This is again not related to the interchangeability of goods.

Hence, the correct answer is option 1 - perfect substitutes.