The correct answer is A. Vertical intermediate stiffener.
Vertical intermediate stiffeners are used to prevent shear buckling of the web in a plate girder. They are typically located at regular intervals along the length of the web, and they help to distribute the shear load more evenly. This prevents the web from buckling under the applied load.
Horizontal stiffeners at the neutral axis are used to prevent flexural buckling of the web. They are typically located at the center of the web, and they help to resist the bending moment that is applied to the web.
Bearing stiffeners are used to prevent local buckling of the web at the supports. They are typically located at the ends of the web, and they help to resist the compressive load that is applied to the web at the supports.
Here is a diagram of a plate girder with vertical intermediate stiffeners:
The vertical intermediate stiffeners are the vertical plates that are located between the flanges of the girder. They help to prevent the web from buckling under the applied shear load.