## **Core Concept**
The question tests knowledge of the comparative nutritional content, specifically calcium levels, in the milk of different species. This involves understanding the composition of milk from various mammals and how it relates to their nutritional needs and physiological adaptations.

## **Why the Correct Answer is Right**
The correct answer, **C. Mare (Horse)**, is right because mare's milk, or the milk of horses, contains less calcium compared to the milk of cows, goats, and sheep. The calcium content in milk varies among species, generally reflecting the growth rate and bone development needs of their offspring. Mare's milk is known for having a lower concentration of minerals, including calcium, compared to bovine (cow) milk, which is often used as a standard reference.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Cow's milk is one of the richest sources of calcium among the common milks consumed by humans, with approximately 120 mg of calcium per 100 mL. This makes it an incorrect choice for the least amount of calcium.
- **Option B:** Goat's milk has a slightly higher concentration of calcium (about 130 mg per 100 mL) compared to cow's milk, making it also incorrect for having the least amount of calcium.
- **Option D:** Sheep's milk is known for its high content of nutrients, including minerals like calcium (around 190 mg per 100 mL), which is significantly higher than that in mare's milk.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the nutritional content of milk can significantly vary between species. This variation is crucial for both the growth of the young and for human consumption, where different milks are chosen for their nutritional profiles. Mare's milk, besides having less calcium, also has a different fatty acid profile and is sometimes used for specific nutritional or therapeutic purposes.

## **Correct Answer:** . Mare