Presence of placenta

The correct answer is option 'C', the presence of placenta is not a characteristic of all mammals. Placenta is a specialized organ that develops in the uterus of female mammals during pregnancy. It allows for the exchange of nutrients, gases, and waste products between the mother and the developing fetus.

Characteristics of all mammals

1. Presence of mammary glands: All mammals have mammary glands, which are specialized organs that produce milk to nourish their young. These glands are typically found in females, but in some cases, they can also be present in males.

2. Endothermy: Endothermy refers to the ability of mammals to regulate their body temperature internally. Unlike cold-blooded animals, mammals can maintain a relatively constant body temperature regardless of the external environment.

3. Presence of hair at some stage of life: All mammals have hair at some stage of their life cycle. Hair is a characteristic feature of mammals and serves various functions such as insulation, protection, and camouflage.

Explanation of the correct answer

The presence of placenta is not a characteristic of all mammals. While most mammals do have a placenta, there are some exceptions. For example, monotremes (e.g., platypus and echidna) are egg-laying mammals that do not possess a placenta. Instead, they have a primitive reproductive system that does not involve the development of a placenta.

Similarly, marsupials (e.g., kangaroos, koalas, and opossums) also have a different reproductive system compared to placental mammals. They give birth to relatively undeveloped young, which then crawl into a pouch where they continue to develop and nurse from the mother's mammary glands. Marsupials have a less developed placenta, or in some cases, no placenta at all.

Therefore, while most mammals have a placenta, its presence is not a characteristic shared by all mammals.

Some mammals are oviparous like Omithorhynchus, they lack placenta.