a
Let amplitude of each wave is \(A.\)

Resultant wave equation

\(=A \sin \omega t+A \sin \left(\omega t-\frac{\pi}{4}\right)+A \sin \left(\omega t+\frac{\pi}{4}\right)\)

\(=\mathrm{A} \sin \omega \mathrm{t}+\sqrt{2} \mathrm{A} \sin \omega \mathrm{t}\)

\(=(\sqrt{2}+1) \mathrm{A} \sin \omega \mathrm{t}\)

Resultant wave amplitude \(=(\sqrt{2}+1) \mathrm{A}\)

as \(I \propto A ^{2}\)

so \(\frac{\mathrm{I}}{\mathrm{I}_{0}}=(\sqrt{2}+1)^{2}\)

\(I=5.8 \mathrm{I}_{0}\)