Contrapositive are the statements in which we switch the hypothesis and the conclusion of the statement and then we take the negation of both.
Here the hypothesis is " A line is horizontal"
And the conclusion is " slope equal to zero"
In order to make the contrapositive statement we switch the hypothesis and conclusion and then take negation.
Therefore, the contrapositive statement is given by
If a line does not have zero slope, then it is not a horizontal line.
Last option is correct.
