$x_1=a \sin \left(\omega t+\phi_1\right)$
$x_2=a \sin \left(\omega t+\phi_2\right)$
$x^{\prime}=x_1+x_2$
$=a\left[\sin \left(\omega t+\phi_1\right)+\sin \left(\omega t+\phi_2\right)\right]$
$=2 a \sin \left(\omega t+\frac{\phi_1+\phi_2}{2}\right) \cos \left(\frac{\phi_1-\phi_2}{2}\right)$
Now as given in question
$2 a \cos \frac{\phi_1-\phi_2}{2}=a$
$\cos \left(\frac{\phi_1-\phi_2}{2}\right)=\frac{1}{2}$
$\frac{\phi_1-\phi_2}{2}=\frac{\pi}{3}$
$\phi_1-\phi_2=\frac{2 \pi}{3}$

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[$A$] The time $\mathrm{T}_{A 0}=\mathrm{T}_{\mathrm{OA}}$
[$B$] The velocities of the two pulses (Pulse $1$ and Pulse $2$) are the same at the midpoint of rope.
[$C$] The wavelength of Pulse $1$ becomes longer when it reaches point $A$.
[$D$] The velocity of any pulse along the rope is independent of its frequency and wavelength.