Final answer:
When a mechanical wave travels from a light slinky to a heavy slinky, most of the wave tends to get reflected back, while some parts of it might get refracted into the heavier slinky. However, without more information, it's unclear if the reflected wave will maintain phase with the incident wave.
Step-by-step explanation:
When a wave, in this case a mechanical wave represented by pulses on a slinky, travels from a medium of one kind to another, it encounters a boundary. What happens at such a boundary is determined by the properties of the media. The incident wave may be partly absorbed by the second medium, it may also be partly reflected back into the first medium, and it is likely also to set up a wave in the second medium. This wave in the second medium is a refracted wave.
However, the medium properties here are the mass per unit length of the slinky coils, identified as 'light' and 'heavy'. When a wave in a light slinky meets a heavy slinky, the wave tends to get reflected at the boundary. Therefore, the correct option is both a) the wave is completely reflected and d) the wave is partially reflected and partially refracted. We are unable to definitively state whether the reflected wave is in the phase with the incident wave without further information. Therefore, options b) and c) may or may not be true.
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