The correct answer is not provided, but we can analyze each option to understand why milk might be considered a poor source of these nutrients.

Option 1: Calcium

Milk is actually a great source of calcium. It is widely known for its calcium content and is often recommended for bone health.

Option 2: Iron

Milk is a poor source of iron. While it contains a small amount of iron, it is not significant enough to meet daily iron needs. Iron is better obtained from sources like red meat, legumes, and leafy greens.

Option 3: Copper

Milk is a poor source of copper. Copper is found in small amounts in milk but is not a significant source compared to other foods like organ meats, shellfish, seeds, and nuts.

Option 4: Sodium

Milk is a poor source of sodium. It naturally contains only a small amount of sodium. People looking to reduce their sodium intake often choose milk as a lower-sodium beverage option.

In conclusion, milk is not a poor source of calcium, but it is a poor source of iron, copper, and sodium.