**Core Concept:** The milk-forming process in mammals involves the absorption of calcium from the mammary gland blood vessels into the milk. The concentration of calcium in milk is influenced by the calcium level in the blood.

**Why the Correct Answer is Right:** Option D (Bovines) is correct because bovines (cows) have a lower concentration of calcium in their blood compared to humans, rabbits, and mice. This is due to the higher calcium-binding proteins, such as fetuin, in the blood of humans, rabbits, and mice, which helps maintain the calcium level in these species.

**Why Each Wrong Option is Incorrect:**

A. Rabbits (Option A) have a higher calcium concentration in their milk because they have a higher concentration of calcium in their blood, allowing for more calcium to be absorbed into the milk.

B. Humans (Option B) have a higher calcium concentration in their milk due to the higher calcium-binding proteins like fetuin, which helps maintain the calcium level in humans and consequently results in more calcium being absorbed into the milk.

C. Mice (Option C) have a higher calcium concentration in their milk because they have a higher calcium concentration in their blood, enabling more calcium to be absorbed into the milk.

D. Bovines (Option D) have a lower calcium concentration in their milk due to the higher calcium-binding proteins like fetuin in their blood, which helps maintain the calcium level in bovines and results in less calcium being absorbed into the milk.

**Clinical Pearl:** The milk of bovines is generally preferred for human consumption because it has a similar calcium content to human milk, making it easier for human infants to digest and assimilate.

**Correct Answer:** .