Which is the graph of the function f(x)= negative square root x

First, note that  is always positive (except for x=0), so must be always negative.

Thus, the only plausible graphs are 1 and 3 since they are below the x-axis.

Now, and are only defined for x≥0, because only for these x'es we can take the square root. 

Note that the third graph has domain (-infinity, 0], so it is not the right one, while 1 is ok.

first graph  

Dhope this