(a) This equation has two harmonic functions of each x and t separately, so it represents stationary wave. (b) This function does not represent any wave as it contains no harmonic function. (c) It represents progressive/travelling harmonic wave as the arguments of cosine and sin functions are same. (d) This equation is the sum of two functions $\cos x \sin t$ and $\cos 2x \sin 2t$ each representing a stationary wave. Therefore it represents superposition of two stationary waves.