To solve this problem, we need to identify the unique characteristics of mammals.$\newline$Mammals are distinguished by several key features, including:$\newline$• The presence of hair or fur on their bodies.$\newline$• The presence of mammary glands, which produce milk to nourish their young.$\newline$• The presence of a pinna, which is the external part of the ear that helps in directing sound into the ear canal.$\newline$Let's analyze the options:$\newline$Option 1: Hairs, tympanic membrane, and mammary glands.$\newline$• While hairs and mammary glands are unique to mammals, the tympanic membrane (eardrum) is not exclusive to mammals.$\newline$Option 2: Hairs, pinna, and mammary glands.$\newline$• All three features are unique to mammals. The pinna is a distinctive feature of mammals.$\newline$Option 3: Hairs, pinna, and indirect development.$\newline$• Indirect development is not a characteristic unique to mammals.$\newline$Option 4: Pinna, monocondylic skull, and mammary glands.$\newline$• The monocondylic skull is not a characteristic of mammals; mammals have a dicondylic skull.$\newline$Therefore, the correct option is Option 2, which lists hairs, pinna, and mammary glands as unique mammalian characteristics.