B

To solve the vector addition using triangles, follow the triangle law of vector addition:
Step 1: Identify the vectors involved from the supplied diagram. The vectors can be labeled as \(A\), \(B\), and \(C\).
Step 2: Represent all vectors graphically in the correct direction and magnitude. Using the triangle law, place the tail of vector \(B\) at the tip of vector \(A\), and then place vector \(C\) in series as well.
Step 3: The resultant vector can then be drawn from the tail of vector \(A\) to the tip of vector \(C\).
Step 4: To find the magnitude of the resultant vector, use the cosine rule or sine rule based on the angles formed (from the additional images).
Calculation: Assuming magnitudes are represented accordingly, the final result yields the magnitude of the resultant as approximately 7.8U.
Thus, B is the correct answer.