(c)
Required area $= \int_{\pi/4}^{3\pi/4}{(\sin^{2}{x - \cos^{2}{x)dx}}}$
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$= - \int_{\pi/4}^{3\pi/4}{\cos{(2x)dx = - \left\lbrack \frac{\sin{2x}}{2} \right\rbrack_{\pi/4}^{3\pi/4}}}$
$= - \frac{1}{2}\left( \sin{\frac{3\pi}{2} - \sin\frac{\pi}{2}} \right) = 1\ sq\ unit$