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