If two waves in the same space interfere completely destructively, which of the following occurs? a. the amplitude decreases. b. the amplitude increases. c. the wave speed increases. d. the wave speed decreases.

The correct option is (A) The amplitude decreases.

Explanation:
The textbook definition of a destructive wave interference is: "Destructive wave interference is a wave interference which occurs when out of phase waves meets with each other and forms a new wave with lower amplitude or zero amplitude."

Now if the amplitudes of the two waves are the same, the amplitude would be zero because both waves would cancel each other out; however in the question, there is no evidence of two waves having the same amplitudes, hence you can assume that two waves have different amplitudes. In that case, the amplitude of the resultant wave will decrease(but will not become zero).