The Most Likely Region of the Radio Spectrum for Communication

The most likely region of the radio spectrum for communication with other civilizations is called "the water hole." What part of the spectrum is this?

A) the part of the spectrum near the radio emission of water
B) the part of the spectrum between the radio emission of hydrogen and oxygen
C) the part of the spectrum between the radio emissions of hydrogen and OH
D) the part of the spectrum near the radio band where water in the Earth's atmosphere strongly absorbs energy