The first and third graphs have a matching, shown in bold (there are other matchings as well). The middle graph does not have a matching. If you look at the three circled vertices, you see that they only have two neighbors, which violates the matching condition \(\card{N(S)} \ge \card{S}\) (the three circled vertices form the set \(S\)).

A bipartite graph with six vertices, in two sets of three. If we call the top vertices a,b,c, and the bottom vertices 1, 2, 3 (left to right), then the graph has the following edges: (a,1), (a,2), (b,1), (b,3), (c,1), (c,3). The edges (a,2), (b,3), and (c,1) are highlighted bold.
A bipartite graph with eight vertices, in two sets of four. If we call the top vertices a,b,c,d and the bottom vertices 1, 2, 3,4 (left to right), then the graph has the following edges: (a,1), (a,3), (a,4) (b,2), (c,1), (c,2), (c,3), (c,4), (d,2). The vertices 1, 3, and 4 are circled.
A bipartite graph with ten vertices, in two sets of five. If we call the top vertices a,b,c,d,e and the bottom vertices 1, 2, 3,4,5 (left to right), then the graph has the following edges: (a,1), (a,2), (a,3), (b,1), (b,3), (c,2), (c,4), (d,3), (d,5), (e,3), (e,4), (e,5). Of these, the following edges are highlighted bold: (a,1), (b,3), (c,2), (d,5), (e,4).