Question

In constructive interference, two waves moving through the same medium must be
1 in phase with each other
2 cancelled out by each other
3 blocked by each other
4 out of phase with each other

Answer 1

First one. Two waves moving out of phase sometimes interfere constructively and sometimes destructively. Destructive interference is when they cancel each other out, constructive interference is when they build on each other.

Answer 2

Thanks :D