c
$A _{\text {resultrant }}=\sqrt{ A _{1}^{2}+ A _{2}^{2}+2 A _{1} A _{2} \cos \phi}$
$\sqrt{3} A =\sqrt{ A ^{2}+ A ^{2}+2 A ^{2} \cos \phi}$
$3 A ^{2}=2 A ^{2}+2 A ^{2} \cos \phi$
$\cos \phi=\frac{1}{2}$
$\therefore \phi=60^{\circ}$
$\therefore \text { Phase difference }=60 \text { degree }$