Consider the three waves z1, z2  and  z3 as z1=Asin(kx−ω t), z2=Asin(kx+ω t)and  z3=Asin(ky−ω t). Which of the following represents a standing wave

Waves Z1=Asin(kx−ωt) is travelling towards positive x-direction.Wave  Z2=Asin(kx+ωt), is travelling towards negative x-direction. Wave Z3=Asin(ky−ωt) is travelling towards positive y direction.Since waves Z1 and Z2 are travelling along the same line so they will produce stationary wave.