D-cards represent cards from various types of bread/baking companies. These were usually packed in loaves of bread or other baking-related items.
Most issues were relatively standard, but some unique D-cards do exist. The D146 Donut Corporation of America cards, for example, were cutouts off of donut boxes.
Most bread issues are relatively scarce by comparison to other more common candy and tobacco cards. These were almost certainly printed in much smaller quantities and practically every bread issue is a somewhat difficult one to find.