Which type of bearing is mounted face to face and back-to-back?

The correct choice is angular contact bearings, which are specifically designed to accommodate the mounting configurations face to face and back-to-back. This design allows these bearings to support both axial and radial loads simultaneously.

In a face-to-face arrangement, the inner rings of the bearings are facing each other, allowing for the accommodation of high axial loads in both directions. Conversely, a back-to-back arrangement positions the outer rings facing each other, providing additional stability and load distribution. This versatility in handling forces makes angular contact bearings a preferred choice in applications where both axial and radial influences are prevalent.

The other types of bearings listed do not classify in the same manner. Conrad bearings typically refer to a specific type of roller bearing that is sealed on both sides and does not have the same mounting configurations as angular contact bearings. Ball bearings generally focus on supporting radial loads and do not allow for the specialized axial load handling that comes with angular contact configurations. Pillow blocks, while having their own mounting advantages and facilitating bearing placements, also do not represent the specific arrangements mentioned for supporting face to face and back-to-back bearings.