Final answer:
The rarity of the 1943 copper penny is due to its steel composition.
Step-by-step explanation:
In the passage, the reader can infer that the rarity of the 1943 copper penny is due to its steel composition. The passage mentions that most pennies in 1943 were made out of steel because of the military needs during World War II. This suggests that the copper penny is ultra-rare because it deviates from the norm.