Which feature of a heating curve indicates a change of state? an upward sloping line a downward sloping line a horizontal line a curved line

The heating curve

The curve is a graphical presentation of the phase of the transition of the substance as heat is added to it. The plateau shows us the mark in the changes in the heat of the substance. The rising temperature stays constant during the phase.

Hence the answer is a horizontal line.

The heating curve is opposite to the cooling curve and it shows the temperature of the substance when the cooling takes place. The heat is refed to the thermal energy and is shown along with the graph in the form a curve.Hence the constant line with no change is a horizontally flat curve.

Thus the option C is correct.

Learn more about the heating curve

link.

A "heating curve" is a graph that shows the temperature of the substance
against the amount of heat you put into it.

For most of the graph, as you'd expect, the temperature goes up as you
add heat, and it goes down as you take heat away.  BUT ... While the
substance is changing state, its temperature doesn't change even though
you're putting heat in or taking heat out.

So that part of the graph is a horizontal line. 

Neptune is the eighth planet from the sun. it was the first planet to get its existence predicted by  mathematical calculations  before it was actually seen through a telescope on sept. 23, 1846. irregularities in the orbit of  uranus  led french astronomer alexis bouvard to suggest that the gravitational pull from another celestial body might be responsible. german astronomer johann galle then relied on subsequent calculations to   spot neptune via telescope. previously, astronomer galileo galilei sketched the planet, but he mistook it for a star due to its slow motion. in accordance with all the other planets seen in the sky, this new world was given a name from greek and roman mythology — neptune, the roman god of the sea.