Concept:The speed of a wave is the product of its frequency and wavelength.Explanation:Distance between two successive crests is the wavelength, λ=0.25m.The particle makes four complete vertical oscillations in one second, so frequency f=4Hz.Wave speed is given by v=fλ.Substitute the values: v=4×0.25=1m s−1.Answer:The correct option is D. 1ms−1.