The correct answer is: A. Transfer the load from the top flange to the bottom one.
A bearing stiffener is a plate or stiffened plate that is welded to the web of a plate girder to support the load from the flanges. It is located at the point where the flanges bear on the web, and it helps to prevent the web from buckling.
Option B is incorrect because the bearing stiffener does not prevent buckling of the web. In fact, it helps to prevent buckling by providing support for the web.
Option C is incorrect because the bearing stiffener does not decrease the effective depth of the web. The effective depth of the web is the distance from the top flange to the bottom flange, and it is not affected by the presence of a bearing stiffener.
Option D is incorrect because the bearing stiffener does not prevent excessive deflection. In fact, it helps to prevent excessive deflection by providing support for the web.