ANSWER: (4), (6)

Reasoning: Interference throughout space is possible with any two waves. If they are not coherent sources, which means their relative phase remains constant, then the pattern will change over time, often rapidly. So, for science and making measurements, you want two (or more) coherent sources. We will learn later how one wave can also produce interference but for now, it's easier to consider two or more. 

*Note: Evan does say coherent sources is required but he clarifies it's required for a constant interference pattern. So, technically, it's not required for spatial interference to be exhibited. 

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