Which formula can be used to calculate the horizontal displacement of a horizontally launched projectile? x = vi(cos ) x = vi(cos )t x = ayt x = vxt (right answer)

Well, the displacement is velocity times time.  The horizontal displacement is then the horizontal component of velocity times time.  The horizontal component is given by the initial total velocity times the cosine of the angle.  If Vi is the initial velocity then choice 2 is right.  It is also possible Vx is meant as the x-component of the total velocity in which the last answer is also correct.  It would help to recreate the problem variables in the math equation editor, or define what you mean by "vi(cos )t"  and "vxt".  These are somewhat vague descriptions.  Hope this helped