The correct answer is: C. Stiffen the structure laterally.
Portal bracing is a type of bracing that is used to stiffen a structure laterally. It is typically made up of diagonal members that connect the top and bottom chords of a truss. Portal bracing helps to prevent the structure from buckling under lateral loads, such as wind or earthquakes.
Option A is incorrect because portal bracing does not transfer load from the top of the end posts to the bearings. This is typically done by the main truss members.
Option B is incorrect because portal bracing does not keep the rectangular shape of the bridge cross-section. This is typically done by the main truss members and the deck.
Option D is incorrect because portal bracing does not prevent the sides-way buckling of the top chord. This is typically done by the main truss members and the deck.