The most likely region of the radio spectrum for communication with other civilizations?

Question

is the water hole, the part of the spectrum

1. Near the radio emissions of water.

2. Near the radio emission of oxygen molecules.

3. Between the radio emission of hydrogen and oxygen.

4. Between the radio emissions of hydrogen and OH.

5. Near the radio band where water in the Earth's atmosphere strongly absorms energy.

Answer 1

Gee whiz, Shawn, do the reading.  It's really pretty interesting.

The answer is 4.

Answer 2

The waterhole is continuously explored with radio telescopes.  The reason for this is that it is thought that all life requires water.  Any alien intelligence would know this, and it gives something like a common ground.  The technical description is from 18cm wavelength (Hydroxil) to 21 cm wavelength (hydrogen).  Hydroxil and hydrogen together make water. It is known as "the waterhole" for obvious reasons.

Answer 3

If those are the only choices, i'd pick #1.

However, we've used lasers for much less time than we've used radio.  We can already send visible light laser pulses that can outshine the Sun by more than a factor of ten.  This could be picked up by a visible light telescope with an appropriate camera.  Both sending and receiving interstellar signals might be cheaper at visible light frequencies.

Here's another strategy.  When your astronomers see a new nearby supernova, send a wide beam in the opposite direction.  Then ET astronomers in that direction looking at the same supernova may notice your signal.  Especially if it is bright and weird. This can be done with radio, too.