Right choice is (b) \(\frac{y_2-y_1}{l_e}\)

To explain: The direct cosines l and m are introduced as l=cosθ and m=cosф. These direction cosines are cosines of the angles of that local x’- axis makes with global x- and y- axes. Let (x1,y1) and (x2,y2) be the co-ordinates of nodes 1 and 2.