It's (a) the Sun. Think of it this way, when we on Earth have a Solar eclipse then the Moon passes between the Sun and Earth. When we see a Lunar Eclipse (the shadow of Earth on the Moon) then Earth passes between the Sun and the Moon, we say that the moon is having an eclipse (a Lunar eclipse). 

So, during a Lunar eclipse if there is an astronaut standing on the Moon looking at Earth he will be in a shadow (eclipse) cause by the Earth blocking the Sun. 

Or think of it another way, an eclipse is when the light source is blocked, Neither the Moon or the Earth give out light only the Sun does, planets and moons just reflect it or block it. 

Hope this explains things for you.
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The astronaut on the moon shall observe a solar eclipse