Unique mammalian characteristics include:

1. Hair or fur: Mammals are the only animals that have hair. This hair helps to insulate the body to maintain a constant body temperature.

2. Mammary glands: These are glands that, in females, produce milk for the nourishment of young ones. This is a characteristic feature of all mammals and is, in fact, the feature that gives this group its name.

3. The presence of pinnae (external ears): Most mammals have pinnae that help to collect and direct sound waves into the ear.

Option B is incorrect because indirect development is not a unique characteristic of mammals; many animals, including certain insects, amphibians, and fishes, undergo indirect development.

Option C is incorrect because a monocondylic skull, in which the skull only articulates with the first vertebra via a single condyle, is a characteristic of some reptiles and amphibians, not mammals. Mammals possess a dicondylic skull, with two occipital condyles.

Option D is incorrect because while mammals do have a tympanic membrane (ear drum), it is not unique to mammals; other vertebrate groups, such as birds and reptiles, also have a tympanic membrane.

So, the correct answer is :

Option A : hairs, pinna and mammary glands.