Final answer:
18 karats means that the piece is made of 75% pure gold, as karat measurement reflects how many parts out of 24 are gold.
Step-by-step explanation:
The purity of gold is usually measured in karats, with pure gold being 24 karats. Therefore, when we speak of 18 karats, we are referring to a gold purity of 18 parts gold out of 24 total parts. This can be calculated as a percentage: (18/24) × 100%, which equals 75%. So when a piece is described as 18 karats, it means that it has a gold purity of 75% pure gold, which corresponds to choice c. 75% pure gold.