Which of the following dimensions can be used for a postcard?

The dimension of 4 x 6 inches is commonly used for postcards due to its standard size that is convenient for mailing and fits within postal regulations for bulk mailings. This size is manageable, making it easy for recipients to handle and is large enough to convey a message or image effectively without being cumbersome.

Other options may not adhere to typical postal standards for postcards. For instance, while 5 x 8 inches can also be used, it is less common than 4 x 6 inches. The dimensions 6 x 12 inches and 7 x 10 inches exceed the typical postcard size, which can lead to higher mailing costs or restrict them from being classified as postcards under postal service regulations. This makes the 4 x 6 inches dimension the most appropriate choice for a postcard.