Answer: The answer is the first diagram.
Step-by-step explanation: We are given two diagrams out of which we are to select the one which shows lines must be parallel lines cut by a transversal.
In the first figure, the sum of two interior angles is given by

which shoes that the two lines which cut by a transversal are parallel. So, this option is correct.
In the second figure, the sum of two angles on a straight line is given by

which shows that a straight angle is of measure 180°, which does not have any relation in proving the two lines to be parallel. So, this diagram is not correct.
Thus, the first diagram is the correct option.