According to triangle law of forces, resultant of two forces must be greater than or equal to the third force. In option (a), In option (a), $ F_{1}(3 {N})+F_{3}(1 {N}) Hence, these sets of concurrent forces may not in equilibrium. In option (b), $F_{1}(3 {N})+F_{2}(5 {N}) Hence, these sets of concurrent forces also may not in equilibrium. In option (c), F1(3N)+F2(5N)>F3(6N) Hence, these sets of concurrent forces may be in equilibrium. In option (d), $F_{1}(3 {N})+F_{2}(5 {N}) Hence, these sets of concurrent forces may not in equilibrium.