Answer:- E) definition of midpoint
Explanation :-
We can look at the second step in the statement which says that" point D is the mid point of AB and E the point of BC " .
And we know that definition of midpoint says that "If a point is a mid point of line segment with end points [tex](x_1,y_1)\ and\ (x_2,y_2)[/tex]
Then the coordinates of the mid point must be
[tex](x,y)=(\frac{x_1+x_2}{2},\frac{y_1+y_2}{2})[/tex]."
And statement 3 say [tex]D=(\frac{x_1+x_2}{2},\frac{y_1+y_2}{2})[/tex]
and [tex]E=(\frac{x_1+x_2}{2},\frac{y_1+y_2}{2})[/tex], where D and E are mid points .
Thus the reason for statement 3 must be the definition of mid point.