Answer: OPTION D.
Explanation:
The formula that is used to calculate the slope of a line is:

Where
and
are two points on the line.
In this case the student finds the slope of the line that passes trough these points:

You can notice that she writes in the numerator that:

So she chose the point
as
and the point
as
.
Then
and
in the denominator must be:

Therefore, she did not keep the order of the points the same in the numerator and the denominator.