Why is it valid to use only $\overrightarrow{F_x}=m\overrightarrow{a_x}$ when analyzing a block whose motion is purely horizontal on a flat surface?

Because weight and normal force always vanish whenever an object moves on a surface

Because Newton's second law applies only to one dimension at a time and forbids vertical analysis

Because horizontal motion means the object has no mass in the vertical direction

Because the vertical forces cancel, giving zero vertical acceleration, so only the horizontal component determines the motion