This is the most important mammalian character of rat

The correct answer is Mammary glands.

Step-by-step Explanation:

1. Understanding the Question: The question asks for the most important or defining mammalian character of a rat (which belongs to the class Mammalia).

2. Analyzing the Options:
- Mammary glands: The presence of milk-producing mammary glands is the primary defining characteristic of all mammals (Class Mammalia), from which the class gets its name. These glands are used to nourish the young.
- Diaphragm: While a muscular diaphragm separating the thoracic and abdominal cavities is characteristic of mammals, the presence of mammary glands is the ultimate defining taxonomic feature of the class.
- Pinna: External ears (pinnae) are found in most mammals, but some mammals (like monotremes and aquatic mammals) lack them or have them reduced, making it a less fundamental defining feature than mammary glands.
- Hair: Hair is also a characteristic mammalian feature, but mammary glands remain the primary diagnostic character that defines mammalian biology and reproduction.

3. Conclusion: Since the name "Mammalia" itself is derived from "mammary" (mamma meaning breast), the presence of mammary glands is the most fundamental, diagnostic, and defining feature of mammals, including rats.