Final answer:
The correct word to fill in the blanks is 'echo,' as it refers to the sound that is reflected off a surface and heard again. When singing in the shower, the voice produces an echo.
Step-by-step explanation:
To echo is to echo with sounds, usually loudly. When you sing in the shower, your voice echoes.
This means that if you stand in a hall and loudly say "hello" you will hear yourself say "hello" a split second later, which is an example of an echo. This can also happen outdoors in a wide-open space with a large reflecting surface nearby, like standing near a mountain cliff in an area with no trees or bushes. Echo is a very useful property of waves, at times utilized in various technologies, like ultrasound equipment where an ultrasound speaker can double as a microphone to record echoes from various depths.