Vector 6 represents F1.
Work Step by Step
F1=(3N)i-(4N) This means that there is a force of 3N pulling to the right, and a force of 4N pulling down. This combination of forces acts to the bottom right (4th Quadrant). Vector 5 is a force with equal x and y component magnitudes, so it can't be F1. The only other vector left is vector 6, which fits F1 perfectly.