Verified Answer :

A zero slope because the gradient is zero

That slope is a B. Zero Slope.
You know this because you measure slopes through the gradient of a line. 
The formula for this is:
Rise/Run
Rise is how far it moves up the y axis, and run is how far it goes through the x axis. 
The rise of a straight line is 0, and the run of the straight line is infinite.
So, you can say that because anything divided by zero is zero, the slope of a horizontal straight line is 0!

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