The correct option is A 2Acosϕ4
x1=Asin(ωt−0.1x)
x2=Asin(ωt−0.1x−ϕ2)
x1+x2=A[sin(ωt−0.1x)+sin(ωt−0.1x−ϕ2)]
=A⎡⎢
⎢
⎢⎣2sin⎛⎜
⎜
⎜⎝ωt−0.1x+ωt−0.1x−ϕ22⎞⎟
⎟
⎟⎠.cos⎛⎜
⎜
⎜⎝ωt−0.1x−ωt+0.1x+ϕ22⎞⎟
⎟
⎟⎠⎤⎥
⎥
⎥⎦
=2Acosϕ4.sin(ωt−0.1x−ϕ4)
So, amplitude will be =2Acosϕ4