In felines, which teeth are the largest and feature a thick mesial root and thin distal root?

The correct choice is indeed associated with the 09s, which refer to the upper fourth premolars in felines. These teeth are notable for their size and structure. The upper fourth premolars have a prominent mesial root, which is robust and provides strong anchorage in the jaw, and a thinner distal root. This anatomical feature allows for effective chewing and grinding of food, as these teeth play a crucial role in the feline's diet, which typically includes meat that requires significant chewing power.

In contrast, the other options do not exhibit the same root structure or size characteristics. Upper canines, while they are significant and prominent, do not have the same root structure as the 09s. Lower molars, although they are also important for the grinding process, do not share the same root thickness differences as mentioned in the question. Lower incisors are much smaller in size compared to the 09s and are designed for different tasks, such as grooming rather than grinding. These considerations highlight why the upper fourth premolars are specifically recognized for their larger size and distinct root structure in felines.