The correct option is C y=acos(kx−wt)
Equation of a wave
y1=asin(ωt−kx)......(i)
Let equation of another wave may be,
y2=asin(ωt+kx)......(ii)
y3=−asin(ωt+kx).....(iii)
If Eq(i) propagate with Eq(ii) ,we get
y=2acoskxsinωt
If Eq(i) , propagates with Eq(iii) , we get
y=−2asinkxcosωt
At x=0, y=0 , wave produce node.
So, Eq(iii) is the equation of unknown wave