A progressive wave travelling in positive $x$-direction given by $y=a \cos (k x-\omega t)$ meets a denser surface at $x=0, t=0$. The reflected wave is then given by
KVPY 2009, Medium
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(c)

Due to hard boundary reflection, phase of wave changes by $\pi$ radians and its direction of travel is reversed.

So, resultant wave is

$y=a \cos (k x+\omega t+\pi)$

$\Rightarrow y=-a \cos (k x+\omega t)$

art

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