A linear pair of angles has a common side, and the vertex is on the line designated by the points denoting the non-common sides.
Consider the first choice:
... the common sided is RL, and the other two points are P and M. Point R (the vertex of the angles) is not on line PM. (In fact, line PM is not shown on the diagram.)
In the second choice, the given angles do not have a common side. (They are actually "vertical" angles.)
In the 3rd choice, the common side is RN, and vertex point R is found on line MO. These angles <em>are</em> a linear pair:
... - MRN and NRO
