Which type of connection can resist vertical forces only?

The correct choice is a roller connection, which is designed to resist vertical forces while allowing for horizontal movement. A roller connection provides support that can carry vertical loads but does not resist lateral forces or moments. This feature is particularly useful in structures where expansion and contraction from temperature changes or dynamic loads need to be accommodated without introducing additional stress.

In contrast, fixed connections are designed to resist both vertical and lateral forces as well as moments, providing a rigid connection between elements. Pinned connections allow for rotation and resist forces in two dimensions (vertical and horizontal) but, unlike roller connections, they can also accommodate some horizontal loads. Therefore, a roller connection is uniquely characterized by its ability to support vertical loads while permitting horizontal displacement without creating restraining forces.