ANS:D - triangulation

For determining the length of a bridge proposed to be built across a wide river, the surveying method of choice would likely be triangulation.

Triangulation involves establishing triangles between survey points and measuring their sides and angles. It is particularly useful for measuring longer distances and covering vast areas, which is often necessary when planning the construction of a bridge over a wide river.

While tacheometry, chain surveying, and hydrographic surveying have their applications in surveying, when it comes to measuring the length of a proposed bridge over a wide river, the method that would likely provide the necessary accuracy and coverage needed for such a project is triangulation.